Macross Missile Massacre: After the station's weapons system overhaul mentioned first at the beginning of Season 4... Torpedoes. Torpedoes everywhere.
MacGuffin Melee: Who Mourns for Morn? With everyone fighting over Morn's latinum.
In "The Sword of Kahless", numerous Klingons believe themselves to be destined to carry it.
Made-for-TV Movie: "The Way of the Warrior", the fourth-season premiere, was the first episode of Star Trek written, filmed, and aired as a TV movie which wasn't also a Pilot or a Grand Finale. The creators took the opportunity to introduce a new character (Worf), redo the main credits, introduce new props, turn two of the Trek universe's allies against each other, and have the biggest battle scene in both TNG and DS9 up to that time.
Madness Mantra: After Ziyal's death, Dukat is reduced to literally being dragged out of Sisko's office while whimperingly repeating "I forgive you...I forgive you..."
Mad Scientist: Dr. Elias Giger from "In the Cards" fits the profile of the more comic, probably harmless type. His theory is that death is caused by "cellular ennui," that cells can literally be bored to death, and so he creates his "cellular regeneration and entertainment chamber" to keep them stimulated. However, his work is supposedly hampered by non-believing "soulless minions of orthodoxy" and he seems badly paranoid that they are out to ruin him and his work.
Frequently used by Cardassian spies, including Dukat himself.
And on Kira by the Obsidian Order, as part of an elaborate scheme to have a Cardassian politician convicted as a traitor and mindscrew Kira in the process.
"Second Skin" may have served as foreshadowing (or inspiration) for Seska, a Cardassian agent appearing as a Bajoran in the Maquis on Voyager. Incidentally, she appeared as a Romulan in one episode.
Also, not really a species change, but wouldn't it be nice if all sex changes could be done as casually and easily as Quark's in Profit and Lace?
Magnetic Plot Device: The wormhole was responsible for nearly everything that went on there. Without it, no one in The Federation or any other race besides the Bajorans and Cardassians would care about Deep Space Nine.
Although they got a TON of mileage out of "just" the Bajorans and Cardassians.
The Main Characters Do Everything: An integral part of Star Trek, but it gets particularly blatant once they get the Defiant. It's common to see the entire command staff leaving the station to go for a ride on the Cool Ship. Kind of makes you wonder who the heck is left behind to handle the daily disasters that normally plague the station.
It's particularly odd when Major Kira, the Bajoran liasion officer to DS9, is out on Starfleet missions with the Defiant and her crew that have nothing to do with Bajor.
Which comes to a head in the sixth season finale "Tears of the Prophets", where Dax stays behind to run the station and gets murdered by Gul Dukat as a result, while Kira boards the Defiant with the rest of the main characters (even Jake) for the invasion of Cardassian space.
One egregious example is the undercover operation to expose Gowron as a Changeling in Season 5. Worf is a Klingon and, in fact, the one who trains the others on how to act. Odo is an expert on Changelings, and he realizes that the actual Changeling is Martok, not Gowron, so he also has to be there. But Sisko's inclusion is questionable, although he's a good enough actor to pull it off and he was the one to suggest the mission to Starfleet Command. But O'Brien? He might have the most combat experience of anyone else in the main crew, but seriously, one would think that Starfleet Intelligence could have sent one operative who could have filled the last slot more effectively.
O'Brien may have been there to supervise their equipment.
Behind the scenes, this was done because actress Terry Farrell's skin condition precluded her from wearing the Klingon makeup, even though her character Dax, as an expert on Klingon culture, would have been a much more logical choice for the mission.
In a villainous example, when the Dominion takes the station, Gul Dukat takes over Sisko's office; Weyoun and the Female Changeling move to the station as well. Given the wormhole, it might make sense for the Female Changeling and Weyoun to be there; however, why is the head of the Cardassian government not on Cardassia Prime, actually governing Cardassia? It either has to do with the Wormhole being probably the most strategically significant position in the Alpha Quadrant, the Founders wanting him in close proximity to them, or he's a sufficiently massive egotist wanting to rub it in the faces of the Bajorans after being humiliated that they successfully revolted under his command.
Man on Fire: Part of Kira's hallucination in "If Wishes Were Horses."
Maybe Magic, Maybe Mundane: The Federation explains away the Prophets (the divinities of the Bajoran religion) as wormhole-dwelling aliens with a strange relationship to time. The Bajorans reframe the Federation's talk of wormholes and aliens in terms of their traditional religious beliefs. The show doesn't really take one side or the other as the truth, and lets each character interpret it according to their own point of view.
Meaningful Name: The Jem'Hadar are named after jemadar, a rank of sepoy (native troops in India who fought for the British Empire and were commanded by British officers). This fits well for a soldier race who are commanded by members of an unrelated race (the Vorta).
For that matter, Feringhee or Ferengi was a disparaging word used by Indians to refer to white Europeans, specifically Portugese traders, who they thought of as greedy, overbearing, and profit-obsessed.
Eris is the name of the Vorta who was "captured" by the Dominion and "rescued" by the Federation so she could spread disinformation about the Dominion. Eris is also the name of the Greek goddess of strife and discord.
Many human Starfleet officers bear the names of significant historical military leaders: witness Commander Benteen and Captain Jellico
Mental World: Bashir's in "Distant Voices" and Sisko's in "Far Beyond the Stars" and "Shadows and Symbols."
Sloan in "Extreme Measures". He tries to kill Bashir and O'Brien by keeping them in his mind as he dies!
Merchant Prince: The Ferengi have this as their Hat. Political power is very much connected to success in business.
Meta Origin: Star Trek has always loved centering its episodes around either the sociological oddities of the various Planet of Hats aliens encountered by the crew and/or a Negative Space Wedgie causing trouble for the ship. DS9 still has these things but ties them into the overall Myth Arc with most of the former being a result of the Dominion's cruel manipulation of the civilizations under its yoke and the latter usually related to the somewhat more well-meaning but often oblivious manipulations of the godlike extradimensional creatures known as the Prophets.
Mexican Standoff: At the end of "Profit and Loss", between Quark, Garak, and Gul Taran.
Also the climax of "Tacking Into the Wind", Gul Rusot points a phaser at Kira, while Garak covers him and while Damar holds them ALL at phaser-point.
Mind Rape: One Bashir-centered episode introduces a race known as the Letheans, who have incredibly powerful psionic abilities. It is stated that the race make for great assassins, because almost nobody ever survives the mind assaults they can attack their targets with. It goes beyond mentally assaulting someone with horrible images to just mentally assaulting someone.
Mind Screw: Benny Russell. The writers considered having Benny Russell hold up the complete script to the series as the very last scene of the series. Boy, that would have been the mind-screw to end all mind-screws.
Mob Boss Suit Fitting: Sometimes played with since Garak, the station's tailor, also has ties to Cardassian intelligence. At one point, Sisko has Garak take his measurements during an officers' meeting in order to pass on information to the Cardassians.
Money, Dear Boy: Averted with Frank Langella's uncredited appearance in the first three episodes of Season 2. He didn't want to make it seem like he was appearing on DS9 for money or exposure.
Morality Pet: Ziyal for Dukat. Dukat seriously dotes on his daughter and he loves her dearly, enough to throw away everything he had built up for himself in Cardassia so he could bring her home. And she does bring out the best in him, keeps him toeing the line of morality and restricting him to Noble Demon territory. She ends up being the only thing keeping his sanity together... which is basically Tempting Fate for a bad ending.
Morton's Fork: Played with in "Treachery, Faith, and the Great River". Damar and Weyoun 7 are faced with a no-win scenario when trying to stop Weyoun 6's defection. If he defects, then he'll pass on intel that could help the Federation win the Dominion War. But if they kill him, then they'll kill Odo (who Weyoun 6 hitched a ride with) and bring down the Founders' wrath. Damar tries to pursue the second option through attempted Loophole Abuse.
Mouth of Sauron: The Vorta serve as the public face of the Founders, who are so paranoid that they rarely venture out of their homeworld. (And almost never in their true shape.)
Dax plays this role from time to time, and she hangs a great lampshade on it in "Trials and Tribble-ations." When the Defiant goes back in time to TOS days, they need to blend in, so they change into Kirk-era Starfleet uniforms.
Bashir: (To O'Brien and Sisko) Aren't you two wearing the wrong color?
O'Brien: Don't you know anything about this period in time?
Must Have Caffeine: The drink of choice for many is "Raktajino"; essentially klingon coffee.
My Country, Right or Wrong: As part of the more nuanced view DS9 takes, a lot of Cardassians are seen struggling with their part of the responsibility for the occupation. This is especially seen in the first season episode Duet.
My Friends... and Zoidberg: Kor belittles Worf by predicting that when bards write a Klingon song about their quest, Worf's verse will consist of, "...And Worf came along!"
Mythopoeia: Bajoran Mythopoeia figures prominently because Bajorans live nearby. Klingon Mythopoeia is there because of Worf and other Klingons and because Klingons are just that cool.
Naval Blockade: Klingons mined the "entire" Bajoran system to prevent Dominion ships from entering the Alpha Quadrant. However because of 2-D Space they only mined a circle around the system rather than the entire system, which would have taken centuries to do anyway.
Starfleet later mined the wormhole in a more realistic 3D fashion, and it was mostly effective until countermeasures could be deployed.
Neck Lift: The new puppet leader of the Cardassian Union, Legate Broca, was neck-lifted by a very deteriorated, yet incredibly strong female shapeshifter.
Shape-shifters don't eat or drink; when Odo tried, it got "messy."
Negated Moment of Awesome: The Dominion begins an insidious takeover of the Federation under the guise of "peace" — just as we've been forewarned — in "The Search". Starfleet capitulates, Jem'Hadar roam free on the station, and even Sisko's subordinates are collaborators. With help from Garak, Sisko organizes a daring insurrection, and ...it's all a holographic simulation. The Dominion was just testing Sisko in order to gauge how he would resist an occupation.
Garak being stopped from killing offf the changelings in "Broken Link". Sure, it was a bit extreme and risked the lives of the entire Defiant crew, but it may well have prevented the hundreds of millions of deaths that happened in the Dominion War, and may have even caused a mass suicide amongst the Jem Hadar and Vorta, averting the war itself.
Never a Self-Made Woman: Kasidy Yates is Ben Sisko's girlfriend, Keiko is Miles O'brien's wife, and Ishka is Quark and Rom's mother. Also, Ziyal would not have mattered to anyone had she not been Dukat's daughter, and Dabo Girl Leeta was only important as Rom's wife. On the other hand, Ezri Dax managed to make the character stand on her own without any men to lean on, and Jadzia did too (she was there well before Worf was introduced, so she's not there as "Worf's wife" any more than he is there as "Jadzia's husband".) And Kira, of course, never depended on any male character for her status, nor did Kai Winn.
Quark's mother is an interesting case because she very much is a Self Made Woman, in fact she's worth more than both her sons combined which is no mean feat in a society where females aren't actually allowed to do business. Then there's her eventual ascension to become the power behind the throne.
But her role within the story is still entirely secondary to her sons'.
Also played with in that while Kasidy, Ishka and Keiko usually appear in the series as the wives/mothers/girlfriends of their men, they clearly have lives of their own and this is occasionally a side issue (Kasidy being a freighter Captain takes her away for long periods of time, Keiko goes on a long term botany study on Bajor, Ishka's "unconventional" lifestyle). None of these women stand still for their men and while they may compromise in the long run they have their own lives and keep them. Kasidy and Ishka also excel in male-dominated fields with little evidence that it was any specific man who inspired them.
Arguably, the Klingons in Season 4 after they invade Cardassia. It sets off a chain of events that climaxes with Cardassia joining the Dominion, granting the Gamma Quadrant empire a foothold in the Alpha Quadrant.
Linked to this, Eddington and the Maquis, who helped drive the final push.
Arguably, Sisko in "Waltz" by putting the idea of destroying Bajor into the now-insane Dukat's head — an act that indirectly gets Jadzia Dax killed by the end of the season. Good going, Emissary.
Played with in "The Maquis" when the Cardassian Central Command's attempts to drive the Federation colonists out of the Demilitarized Zone results in the epoynmous group's creation. However, rather than improving things for our heroes, the Maquis running loose ends up making the geo-political situation between the Federation and Cardassia even more precarious.
However, it also gets played straight in the same episode when Central Command throws Dukat under the bus and blames him for the DMZ situation. Rather than saving face, it instead convinces Sisko that the Maquis are right about the weapons shipments and he proceeds to rescue the Gul. Dukat is NOT happy to learn about being scapegoated and helps expose the weapons shipments partly to preserve the treaty and partly to spite Central Command.
No Gravity for You: One character of the week from a low-gravity planet is able to wipe out several hostage-takers by using this trope.
No Hero to His Valet: Kor's last appearance. Aside from his descent into Perilous Old Fool, Martok harbored a deep grudge against Kor for getting him blacklisted by the military for the sole reason that he was not born to a noble house.
Noir Episode: "Necessary Evil" is Noir to its very bones. After a priceless opening sequence between Quark and a Femme Fatale in her house, amid a thunderstorm...the episode has Constable Odo using the Private Eye Monologue to parody the Captain's Log, and flashbacks to a murder he investigated while working for the Cardassian occupiers. Dark shadows and dim lighting abound, and Quark shows a surprisingly cool head when being threatened at gunpoint by a Mook. All that's missing is appropriate music, and maybe Odo and Quark wearing fedoras and trenchcoats...though Odo's attire in the flashbacks might pass for the coat. Interestingly enough, Major Kira turns out to be the real "Femme Fatale" murderess.
No Man Should Have This Power: The end of "The Sword of Kahless". Jadzia beams the sword out into deep space to prevent anyone from squabbling over it.
No Name Given: The Female Changeling. In fact, none of the changelings from the link seem to have names, but the female is the only recurring one and so is the most obvious. Only Odo who was given a name by his discoverer and the other 'lost' changeling have names because they were raised outside of the link. Presumably when you spend most of your time physically co existing with the others of your own kind, names are somewhat irrelevant.
Odo: You haven't told me your name.
Female Changeling: What use would I have for a name?
Odo: To differentiate yourself from the others.
Female Changeling: I don't.
Odo: But, you are a separate being, aren't you?
Female Changeling: In a sense.
No OSHA Compliance: Many of the repeat offenders from the rest of the franchise make appearances here, though they are not always the same because of differences in this show's setting and technology. Many instances of this trope can be handwaved by the station's Cardassian design, which is shown to be much less safety-oriented than the Federation's.
In Civil Defense, the station's entire life support system is taken offline by destroying a single control station. Though it was intentional in this case, it would obviously be a HUGE safety issue in any other circumstance.
Cardassian force fields are evidently lethal by design, something the Federation and Bajorans corrected immediately when they took control of the station. Particularly nasty, considering they are invisible to the naked eye.
The entire ore processing operation on Deep Space 9, back when it was still Terok Nor, was full of this trope, since during the occupation it was used by Bajoran slave laborers whose health was the farthest thing from the Cardassians' minds.
The Cardassian attitude toward safety is underscored when Gilora, a Cardassian scientist, sees that O'Brien had installed a second life support backup for the station to comply with Federation standards. Gilora thinks it is unnecessary and inefficient, while O'Brien is surprised that she would be comfortable with only one backup.
Non-Linear Character: The Prophets or the Wormhole Aliens. Sisko had to teach them linear time in the pilot.
Noodle Incident: Pelios Station. Apparently, something happened there involving Curzon, Benjamin, and a dancer, but Benjamin's embarrassed enough by the story that whenever it's brought up in public he immediately cuts Dax off.
Whatever it was that happened with Dukat's father that Garak had a hand in. All we know is that he trusted Garak, and ended up on trial.
Also whatever caused Garak's exile. We know that Tain felt Garak betrayed him, but Garak responds that he never betrayed Tain, "at least, not in my heart." Garak gives several different versions of why he was exiled over the series (not counting the obviously false ones like "tax evasion"), including letting some Bajoran street urchins go on the eve of the withdrawal, and having a shuttle containing a Bajoran terrorist shot down, killing the son of a high ranking official who was also on it.
Not a Date: After Ziyal first asks Garak to accompany her to a holosuite recreation of a Cardassian sauna, and he has visions of her motive being to kill him and present his head as a birthday gift to Dukat, Quark asks if he's going to cancel the date. Garak protests that it's not a date.
Not So Different: Once we find out that Odo's people are the Founders, its hard not to see that Odo's inherent need to maintain order derives from the very thing that led to the Dominion being created in the first place.
Eddington gives a great "The Reason You Suck" Speech to Sisko about the nature of the Federation (conveniently glossing over the fact that he and many other former Starfleet officers in the Maquis, explicitly betrayed the Federation by stealing resources, technology, and weapons when they defected, rather than resigning their commissions to take up the "noble" cause).
Eddington: Why is the Federation so obsessed with the Maquis? We've never harmed you. And yet we're constantly arrested and charged with terrorism. Starships chase us through the Badlands, and our supporters are harassed and ridiculed. Why? Because we've left the Federation, and that's the one thing you can't accept. Nobody leaves paradise. Everyone should want to be in the Federation. Hell, you even want the Cardassians to join. You're only sending them replicators because one day they can take their "rightful place" on the Federation Council. You know, in some ways you're even worse than the Borg. At least they tell you about their plans for assimilation. You're more insidious, you assimilate people and they don't even know it.
The Maquis and the Bajoran Underground share a lot of similarities, as both fought to defend their homes from the Cardassians. Part of the Blue and Orange Morality of Deep Space Nine is that overall the Federation tends to consider the Bajorans as having been freedom fighters, even though Kira herself admits that they often veered into terrorist acts, while they condemn those very same qualities in the Maquis and label them as terrorists.
Part of the reason for that is likely the fact that, unlike the Bajorans, the Maquis are former Federation citizens. Thus, every time they pull off some underhanded terrorist act, it makes the Federation look bad. So the Federation's chasing after them so they can say "Hey, look, we don't like what they're doing, either." Makes sense when you consider the highly political nature of the Federation.
Rom and Sisko in "The Ascension", when they discuss Nog and Jake's failed reunion and how their kids lack each other's qualities and should learn from each other. They just sit down and have a hearfelt conversation like two equals who are both concerned fathers.
Not So Omniscient After All: When first introduced, Enabran Tain appears to be The Omniscient. Even in retirement, he knows everything that's happening. He even knew Bashir would come to visit him and what Bashir's food/drink preferences were, despite it actually being an impulsive, last-minute decision from Bashir's point-of-view. Later on, we're shown a different side that fits this trope. When he leads a joint Obsidian Order/Tal Shiar task force, he didn't realise it was a Founder created plan or that his second-in-command was a Founder in disguise. He even mentions to Garak that he's clearly lost his touch and that he would never have been so easily fooled prior to his retirement.
Not So Similar: Q makes this observation about Sisko and Picard when he finds Sisko is a lot easier to play with, though more likely to hurt him if he's careless.
Not So Stoic: Of the regulars, Odo; Captain Solok from "Take Me Out to the Holosuite".
Weyoun was sort of his own Nth Doctor, as he was killed and cloned several times over the course of the series, with only slight variations to his personality.
The Defiant is destroyed at one point. An episode or two later an identical model of the ship is delivered and simply renamed "The Defiant" (in defiance of the Trek tradition which would have at least named it Defiant-A).
Starfleet rarely uses the alphabetical suffixes for namesake vessels according to Word of God; typically only allowing them for starships named Enterprise in honor of the original vessel's accomplishments. The second Defiant was originally intended to be an exception to this rule in honor of the first, but someone on the production team made a mistake and gave the new ship a new registration number, and the Defiant-A never was.
The rule appears to be that it gets a letter suffix if the registration number is reused, and may or may not get a numerical suffix if not. A numerical suffix appears to be used if another ship of the same class gets the same name but not the same number, and not used if the name goes on a ship of a different class.
The Defiant in particular is an odd case, being equipped (like the NX-01) with an NX registry number prefix, denoting a prototype. As the new Defiant was not a prototype ship, the registry number being changed might have been a result of that, if it hadn't been a mistake.
Obstructive Bureaucrat: Odo finds that acting this way—or threatening to—is a good way to keep Quark in line. He also complains occasionally that Starfleet regulations are too bureaucratic.
Odd Friendship: Odo and Quark (though Odo refuses to admit it, even in the finale).
O'Brien and Bashir to a lesser extent. When they meet, they certainly are this. O'Brien is an enlisted man, realistic, pragmatic, his ideals worn down by the experience of realities of war. Bashir is a young, brash officer, arrogant and pompous who believes in lofty ideas and longs for adventure. As time goes on, they rub off on each other and become less of an odd couple.
Oddball in the Series: A combination of factors worked against DS9 in establishing its own legacy. Keep in mind that DS9 ran concurrently with both TNG and VOY, and ultimately got lost in the shuffle. Those not familiar with Trek will likely have not heard of DS9. Kids and young adults were more likely to bond with Voyager because of its episodic nature and iconic elements. And Trekkies are liable to dismiss the show entirely due to its lack of exploration. Finally, the arc-based format does not lend itself to syndication.
Of Corpse He's Alive: The Magnificent Ferengi - The Ferengi accidentally kill their Vorta hostage, Keegan, so Nog uses neural stimulators to make the corpse walk. Nog makes him lurch awkwardly and run into a wall, but the plan still works.
Offscreen Moment of Awesome: In "Soldiers of the Empire", Martok, Worf, and Dax join a Klingon ship, the Rotarran, on a mission to discover a missing Klingon vessel that disappeared near Cardassian space after the Cardassians aligned themselves with the Dominion. Much is said over the fact that the Rotarran has lost every engagement it has been in with the Jem'Hadar. When they finally face a Jem'Hadar ship in combat, we don't get to see the battle, the outcome of which is the Rotarran's first victory over Dominion forces.
Kor and a team of six Klingon volunteers in a lone, damaged bird-of-prey holding off a Jem'Hadar fleet long enough for the Klingon task force to get away.
Worf's face in Accession when he finds out that Keiko is pregnant again. O'Brien relates how Worf was forced to help deliver Molly when the Enterprise was damaged and adrift after colliding with a quantum filament. On noticing his panic, Bashir and O'Brien mock him accordingly.
Bashir: Oh, well...I'll be sure to call you when she's ready to deliver; you can lend a hand.
Worf: Seven months? Unfortunately I will be away from the station at that time. Far away. Visiting my parents, on Earth. (Beat) Excuse me. (Quickly leaves).
Worf and Garak after stumbling into a Jem'Hadar invasion fleet during In Purgatory's Shadow.
Garak: Maybe this wasn't such a good idea after all.
Followed by the crew of DS9 seeing the same fleet pour through the wormhole.
In The Die is Cast, Enabran Tain's reaction when he's told that his fleet of 20 Romulan and Cardassian ships is surrounded by 150 Jem'Hadar warships.
Oireland: Normally, Colm Meany's Irish accent is barely noticeable. Sometimes, however, O'Brien will start waxing poetic about Ireland or something related to it, and begins slipping into this trope.
Old Shame: Inverted, Terry Farrell regretted leaving the show for the seventh season for a role on Becker because she loved playing Jadzia, calling her a "superhero."
Omnidisciplinary Engineer: O'Brien seems to know all about replicators, power systems, ship drives, mechanical systems of all kinds, weapons, and computer programming, which is completely different from any type of mechanical engineering.
Omnicidal Maniac: Dukat becomes one of these over the last two seasons of the show.
Ominous Floating Castle: For all intents and purposes, this is what DS9 (or "Terok Nor") represented to the Bajoran people during the occupation. It wasn't until the discovery of the wormhole that Sisko moved the spiky eyesore out of their orbit.
Terok Nor is arguably even worse in the Mirror Universe, where it's still sitting on Bajor like a vulture. The bottom levels are composed of a hellish forge running on slave labor.
The villain Falcon (the image of Chief O'Brien) grants agent Komoninoff (Kira) and Bashir one last kiss before he shoots them. Bashir proceeds to remove her earring while they kiss and tosses it to the floor to make it explode, distracting Falcon and his minions long enough to knock them out and escape.
Bashir and Garak are strapped to a giant laser. Bashir's last request is for Dr. Honey Bare (Dax) to let her hair down (she's very shy, but she does it). This is enough seduction for her to slip him the key.
Oppressive States of America: The Bell Riots episodes feature The Sanctuary Districts, sections of American cities walled off that housed the poor and unemployed. While their intent was to aid them, they later degraded into interment camps.
Orphaned Punchline: Quark does this twice, in the first-season episode "The Nagus" and season four's "Homefront", both times while talking to Morn — and both times, the joke has an Andorian in the punchline. Quark has to prompt Morn with "get it?" in the first example, and the second example completely eludes Morn.
Quark: Then the Andorian said, "Your brother? I thought it was your wife!"
Quark: So then, the Andorian says, "That's not my antenna."
The Other Darrin: Two different people played Quark's mother Ishka, and Tora Ziyal was played by three different people.
Though apparently Klingons physically mature faster than most races... Alexander would have been about 8 years old when he began military service. Being 1/4 human didn't seem to matter any considering future Alexander seen on TNG looked fully Klingon regardless of minor human genetics.
Outgrown Such Silly Superstitions: The greatest aversion of the trope across the entire franchise. Religion is treated very even-handedly, showing the good and bad of all sides and portraying major characters across the broad spectrum of belief from sincere (Kira, Vedek Bareil) to fanatics and opportunists (Kai Winn).
Out-of-Character Alert: In "Armageddon game", Keiko figures out a video has been tampered with because Chief O'Brien seems to be drinking coffee in the afternoon.
Ends up being a subversion when it turns out at the end of the episode that O'Brien does drink coffee in the afternoon sometimes.
How Odo realizes Martok, not Gowron is a Changeling in "Apocalypse Rising."
Out with a Bang: Curzon Dax apparently dies of old age in the pilot. Years later, we learn that he was Jamaharoned to death by Vanessa Williams.
Pimped-Out Dress: Kira and Dax wear a couple when they were in a King Arthur program on the Holosuite. When they meet Worf, Kira is mortified but Dax doesn't seem to care. Worf merely compliments Kira's headdress.
Pint-Sized Powerhouse: The Defiant, first of her class. Officially roled as an "escort ship", a look at the class's stats quickly establish it as dyed-in-the-wool warship... that's barely five decks tall. The Defiant herself can give as hard as she can get, thanks to a variety of new martial technologies the class is a testbed for.
Placebotinum Effect: Odo inadvertently traps himself, Sisko, Garak and Dax in his own memories ("Things Past"). It was later revealed to be a variation on 'linking' between Changelings, amplified by Odo's runabout flying into a plasma field.
The Plan: The Dominion pulled off several of those.
Planet of Hats: While still the same part of Star Trek that it always is, DS9 went deeper into the cultures of some races far more than any other Star Trek. Cardassians and Klingons got a lot of development, but even the Ferengi managed to put a few good words in for themselves.
Subverted most obviously with Rom and Nog, two Ferengi who stink at being capitalists, but blossom as an engineer for the Bajoran Militia and the first Ferengi in Starfleet, respectively.
Nog did show capitalist talents, but chose to become an officer instead.
The Federation's "hat" also gets subverted, as DS9 makes it clear that lots of people don't think it's utopia — and they're often right.
DS9 also revealed that the Federation itself is seen as a Planet of Hats in by some non-Federation races. Exactly how certain alien cultures view the Federation is illustrated when Quark and Garak discuss their opinions of the Federation over some root beer.
Quark offers some root beer to Garak, who tries it and gags. Garak: It's vile! Quark: I know. It's so bubbly and cloying... and happy. Garak: Just like the Federation! Quark: And you know what's really terrifying? If you drink enough of it, you begin to like it. Garak: It's insidious. Quark: [Smirks] Just like the Federation.
And whats one of the first things Nog does when he joins the federation starfleet academy, order a root beer.
Justified in the case of the new races from the Gamma Quadrant they encounter, as most if not all of them were manipulated into a specific form by the Dominion in some way either to serve a specific function for them or as The Punishment.
The Dominion's "hat" is to embody the traits of the Alpha (and Beta, but Deep Space Nine tended to forget that) quadrant powers to their worst conclusions:
The Dominion likes to preach peace through diplomacy and friendship like the Federation, but membership in the Dominion amounts to indentured servitude and slavery. And since the Dominion was designed as an anti-Federation, it does the same thing with another trait: both the Federation and the Dominion consist of multiple species working together for what they see as the common good, but where the Federation has equality as a guiding principle the Dominion is based on giving specific species specific tasks under the Founder's undisputed rule.
The Founders believe themselves to be superior to all other races and prefer to engage in espionage, infiltration and subterfuge to undermine other powers rather than resort to military force just like the Romulans, except that the Dominion will go as far as blowing up stars in the center of homeworld systems to wipe out its enemies.
The Jem-Hadar are the perfect soldiers who will willingly die by the millions to win a battle, like the Klingons. But unlike the Klingons, the Jem-Hadar are cold and machine-like in combat.
The whole culture of the Dominion mirrors Cardassia with its subjects willingly loyal and dutiful to the state, yet they are enslaved anyway through fear and through addictive drugs.
Platonic Life Partners: Sisko is fond of calling Dax "Old Man." He was a close friend of the male Curzon Dax, and the friendship carried over when the symbiont was transferred to Jadzia and Ezri — both female.
Playing Cyrano: Worf trains Quark in how to woo the Klingon Grilka, the Ferengi's one-time wife. Worf is infatuated with her himself, but he is an exile and cannot pursue her.
Playing Gertrude: In "Dax," Ilon Tandro is played by Gregory Itzin, born in 1948. His mother, Enina Tandro, is played by Fionnula Flanagan, born in 1941.
Politically Correct History: Sisko doesn't like Vic's casino program because it's set in a Politically Correct History version of 1961, and as such is an insult to those oppressed in that era. He points out that at that time African-Americans could be janitors or entertainers for the casino, but never customers. His future wife looks at the program as a representation of What Could Have Been rather than a misrepresentation of history.
This may have also been a callback to Sisko's experiences as Benny Russell in Far Beyond the Stars, illustrating that Sisko has a more personal connection to the racism of the past than other 24th century humans.
Poor Communication Kills: The show does a decent job of averting this one, but "Till Death Do Us Part" is a real exception. Sisko tells Cassidy how the Prophets told him in a vision that he couldn't marry her, and he accepts that, but she's devastated. He could have mentioned how he ignored one of their warnings before, and that it was directly responsible for the death of Jadzia and the rise of Gul Dukat as a dangerous Pah Wraith cult leader, and that he knows that to ignore their warning again would spell disaster, maybe even for her. But he doesn't say anything of the sort.
Poorly Disguised Pilot: Averted with "The Maquis". The duology sets up the renegade Federation colonists and the Badlands, but does not feature any of the characters or ships that would be featured on Star Trek: Voyager. And even though the Maquis were introduced specifically to set up the plot for Voyager, they ended up playing a more important role in DS9 anyway.
Post Mortem Comeback: "Trials and Tribbleations" features the Klingon from the Star Trek TOS episode "The Trouble with Tribbles" going back 100 years into the past to plant a bomb that will kill Kirk.
Power Creep, Power Seep: Often seen with Odo and Changelings in general: to take one example, the Mirror Universe Odo was killed with one phaser shot, while in "Apocalypse Rising" it took dozens of Klingon disruptor blasts to bring a Changeling down. Some of this can be explained away by Odo being less experienced than the Founders.
Expanded Universe sources also usually give phasers a lot more settings, including many that are far more lethal than anything hand-held disruptors are normally capable of. The most lethal thing we've ever seen a disruptor pistol do was take a few seconds to disintegrate a target, while we've seen hand phasers disintegrate a larger than human target instantaneously.
Pragmatic Villainy: the aforementioned Ferengi. They'd never practice mass slavery or genocide—because people who are enslaved and/or dead can't buy things.
This argument also helped push forward feminism, but only very late, after millenia of hardliner male chauvinism.
The Dominion occupation of Bajor definitely fits this. We know the Founders are willing and able to do far nastier things than merely murder entire planets full of people, but they carefully treat Bajor with restraint and respect, since they hope to convince planets to join willingly. Quark even points out how different it is than the Cardassian occupation was.
The Precarious Ledge: The show uses the same "narrow ledge inside a cavern" set for this in several episodes.
In "Move Along Home" getting past one of these that's being shaken by an earthquake is the final challenge in the Wadis' board game. They fall off into the abyss, but fortunately you can't really die in the game and they appear back in the real world.
In "The Sword of Kahless" Jadzia, Worf, and Kor encounter a narrow ledge while working their way to the surface from the Hur'q treasure room. Kor falls down but grabs hold of the edge, and the others pull him back up.
Premature Eulogy: Occurs as often as would be expected from a Star Trek work. Some episodes are practically made of this trope.
Pre-Mortem One-Liner: One of the best ever by Dukat, addressed to Kai Winn. He says it with so much quiet venom you can almost see it physically dripping from his words: "Are you still here?" His Bond One-Liner after is just as vicious, "Farewell, Adami."
Pulled From Your Day Off: In "Defiant", Gul Dukat laments to Ben Sisko that, rather than spending the day trying to stop the hijacked USS Defiant from attacking Cardassian space, he was supposed to be taking his son to an amusement park for his eleventh birthday.
Dukat: He always wanted to go, but I never had the time. I told him, "This year will be different, Mekor. This year I will make the time." Sisko: I had the same experience with Jake. At that age, they never understand, do they? You just hope that, one day later, they'll look back and say, "Now I understand. Now I know why he did that." Dukat: When my son looks back on this day, the only thing he'll remember is that a Federation officer, on a Federation ship, invaded his home, and kept his father away from him on his eleventh birthday, and he won't look back with understanding. He'll look back with hatred, and that's sad.
Put on a Bus/What Could Have Been: Romulan Subcommander T'Rul is brought in in the season-three premiere, "The Search", to operate the cloaking device. After "The Search, Part II", she is never seen again, because the writing staff couldn't think of anything to do with her. Though she was certainly abrasive and rather one-dimensional in this episode, one could think of this as being a missed opportunity; there still has not been a regular Romulan crewmember on a Star Trek series.
Putting on the Reich: To a degree with the Cardassians, but utterly explicit during the heartbreaking episode "Duet."
Pyrrhic Victory: The Female Changeling decides to make the Federation's victory "taste like defeat." It's a Bolivian Army Ending for her, since she thinks the Changelings are about to become extinct.