Each character gets at least one, but Black Mage, as the resident Chew Toy gets the most of them. It doesn't help him that his face is EASILY the best suited for these expressions.
The universe itself goes Oh, Crap! when Black Mage becomes the ruler of Hell.
In "Doc Gets Rad": A couple of mooks in a helicpter are laughing at the Doctor as they think he's about to die from jumping from a ledge on a motorcycle. Then they realise he's really flying right at them...
King Radical, when he hears about Dr. McNinja's new motorcycle.
It's very subtle, but the last panel of this page is an excellent example.
Gordito gets a big one in the second-to-last panel when he realizes his mustache is useless against the principal... and he's going to have to shave it.
In Agents of the Realm, Jordan reacts like this when she realizes that Norah's with her in the movie room.
In "World's Finest", Alfred and the Kents get together to eat sweets and commiserate about their experience with parenting superheroes. After Alfred mentions that Clark could be eavesdropping, Martha yells that her son would never do something like that. Cut to Bruce and Clark in the Bat-cave; Clark is sweating as he freaks out about the fact that his mom caught him and Bruce is unfazed.
Clark: Oh no, she's onto us.
In "Shovel Talk, Duke accidentally lets it slip that Cassandra is seeing a boy; he starts sweating as he begs Tim to drop it. When everyone starts to grill him a week later, he says Cass will kill him.
In "Online", Barbara finds out that Jason has a Tinder-expy account and starts dropping lines from his dating profile when he comes over for help with something. It takes him three lines to realize that she discovered it, but then he gets wide-eyed and starts sweating.
Rob Ten Pas' Bomango comics tend to feature a number of these, given the combination of instability and amazing strength of the title character. But the last panel of this strip has Gogo herself giving the look, and with good reason. Andy joins in as well as he witnesses what the girls have unleashed...
In Consolers, Atari has one right before she gets buried under tons of unsold games (most of them E.T) and consoles.
Crimson Knights: The knights have a collective one when one of the cannons of the Cralesian army opens fire on Brennus Cliff.
Dan and Mab's Furry Adventures has its share of Oh Crap moments, particularly in the latest arc. One of the better ones has to be Regina's, starting on page 973. Of course, this being DMFA, it's the page's punchline.
Regina:(thinking) Should be easy. I nearly pulped him last time we fought. I mean, it's been what, six years? What could he have learned in that time? * K-shnk! K-shnk!* Dan: I learned to hear thoughts. Among other things. Regina:(thinking) OK. New plan. Piddle self. Run.
U16 Piccolo gets one when he realizes that Vegetto is injured... which has never happened before.
Jeice gets a BIG one when it's announced that he has to fight Frieza. Yes, that Frieza. Needless to say, he immediately gives up.
U13 Vegeta looks more than shocked when Dr. Raichi summons an army of SAIYANS after the former easily disposes of the Ghost Warriors Frieza and Cooler.
Rikashar also has a Freak Out after Ariel beats the crap out of him when he tries to abduct her and make her his wife with his reaction following here, and the last panel of the same page is more of an "Aw, shit" after he sees who's caught up with him.
Yet another from the same comic happens in the seventh panel of this chibi-page, which parodies events from that same chapter.
In Earthsong, Willow has one when she's walking through the woods at night ... something tugs on her cloak ... and she looks up, right into the teeth of a hungry vampire.
A minor one in El Goonish Shive: Nanase uses her fairy doll to contact Ellen, and tries to wake her up by tickling her nose with her tail. Her reaction when she realises she's about to get messily sneezed on is hilarious.
There's a slightly more major one here. More of an awkward "oh crap" than a dangerous one, but just as expressive...
Charlie has the distinction of getting one offscreen during this exchange with Parson over the Eyebooks, after just getting Parson to use one of the calculations he owed Charlie to assess whether it was worth giving up the rest of the calculations in exchange for information from Parson.
Charlie: That was clever though, right? Parson: Sure. Determining the decision not to spend all the calculations. Cost: one calculation. Charlie::) Parson: Teaching me that my bracer can make sophisticated predictions about the future? Priceless. Charlie:...:(
Later on, pretty much all of Gobwin Knob (especially Stanley and Wanda, who had been unshakable beforehand) get this when Faq's Turnamancer linked with Charlie manage to force them to end their turn.
When Lilith tells Parson that the best way out of a bad situation is for her to commit suicide, she sends him an image of using one of Charlie's guns to kill herself. Until this moment, Fantasy Gun Control was strictly in place. Parson has to take a minute to digest the implications of that image.
Sarine and Jon go through so much together in Errant Story that it's all but inevitable that they'll share at least one such moment, and sure enough, they do.
In Exterminatus Now, a random paranoid tries to temporarily ingratiate himself in the protagonists so he can hide from "government spooks". The main characters proceed to inform him that those people are likely just foreign businessmen, while revealing enough information to make the guy realize they are government spooks.
And here's the reaction of the original team members when Rogue horribly insults Lothar.
When the castle decides to put out the fire in one of it's room by using the water from it's cistern, the cast that is standing right underneath it has only a moment to have a wide eyed This Is Gonna Suck reaction.
When Professor Tiktoffen realizes that Agatha chose a blunt weapon instead of a knife for a very good reason.
Martellus is yammering on about how dangerous and unacceptable it would be if Dr. "Dim"'s bears were to actually follow their intended master which is meant to be the doctor's masterpiece to Krosp, who is to Marellus' horror addressed by one of said bears as "master" revealing that he is the doctor's masterpiece.
Martellus had one earlier when the Corbettites' cook/general, in the middle of the kitchens, worked out where the Beast would emerge based on the idea that "there's no other large source of metal" it could get to easily. Cue wide shot of them surrounded by ovens, stoves and cold stores...
A good chunk of the cast immediately has a quiet freakout upon realizing exactly why the seemingly kindly Dr. Vapnoople had been lobotomized by the Baron, moments after the lobotomy's undone.
Later, Thacogets one when Goblinslayer illuminates the darkened sewer, negating Thaco's single advantage, (his ability to see in the dark) resulting in a straight up fight between the two adversaries that Thaco cannot possibly win.
In the same fight, Goblinslayeralso gets this, when Thaco remembers he is a Monk and can snatch arrows, making Goblinslayer's ranged attacks useless.
Saves a Fox's entire self-image was that people are not bound by their destiny. When the fated day came when a fox got stuck in her home village's food supply, she snapped its neck in order to spite her fate, and saved the tail as a memento. That's not the Prophecy Twist resulting in her Oh, Crap! moment... Dies Horribly informs her that the symptoms she described the fox having was that of Sunset fever, something that causes animals to become delirious before dying a horrible, painful, and silent death. Saves A Fox gave it a Mercy Kill before it could suffer the worst effects of it. The look on her face that not only is her entire self-image a lie, but her friend whose name reflecting his destiny is "Dies Horribly" is not able to escape his fate either tells us she's heading right into Heroic BSoD territory.
The Psionic Minmax alt was supremely confident until he realized Kin announcing his intent to kill everyone in the Maze of Many had united all of the survivors against him.
In Godslave, Edith has this reaction when she realizes that a priceless jar she has just performed Desperate Object Catch for is about to break after all.
Sniper: So why didn't you become a group leader? West: Don't know. They basically just asked my kill count. Sniper: Your kill count? [Beat]Oh. My. God!
Sydney of Grrl Power gets one, when testing her powers with Maxima.
Sydney: What am I supposed to do. Maxima: Just survive. Sydney: Okay. Wait, WHAT?
In Guilded Age, Harky has an impressive one when he realizes Byron is The Berserker, making a quick turnaround from mocking the heroes trying to fight him to shouting for someone to get him out of the pit and shoot Byron down.
This trope is often used in Heart Core nearly every time that someone manages to piss off Ame, mostly during fights. But of course, as you can see in the image above, she's not immune to this trope herself whenever the enemy pulls out a card of their own.
In Heritage, the mysterious Jack is understandably surprised when the otherwise ordinary hero Frate suddenly gains super angel powers in the middle of a fight.
Jack: I was going to report back if you turned out to be more than meets the eye, but n— UURGH! HOLY S#* @, YOU GOT FAST ALL OF A SUDDEN!
Eridan's reaction to his victim Kanaya coming back as a glowing super powerful Rainbow Drinker (the Troll version of a vampire) that just snapped his wand leaving him unarmed while she revs up her chainsaw is priceless.
Even better and even more cathartic? Bec Noir's reaction to seeing PM arrive in the troll's session, wearing the White Queen's ring and looking pissed. Off.
Two from two different characters, one after another. The first is when Rose realizes that it's not a memory when the Draconian Dignitary is in Dave's room with her. The next is when Dave remembers that he's not in his room. He's still asleep on Derse.
Aradia's reaction to seeing Sollux is as cheerful as you'd expect from two friends (or more) meeting up for the first time in real life... right until she realises he's Brainwashed and Crazy.
A three-way version occurs when Jake and Jane stumble into Derse right before it starts falling apart and Dirk comes across them. All three of them proceed to freak out.
Though stoic, the music and editing made it clear that Davesprite got one of these when Jack was prototyped for the fourth time in the middle of their fight.
Kill Six Billion Demons: Defied in "King of Swords", when Allison realises she isn't going to survive the fighting arena because she just spent all her strength while drawing everyone's hostile attention. Her thoughts do go "Ah, shit," but her face only shows calm acceptance as she remembers the Zen-like teachings of her master.
The Less Than Epic Adventures of TJ and Amal: TJ freezes after telling Amal to go ahead with a "bedtime story" when he realizes Amal was talking about the books on tape, the container for which actually has no tapes but is where TJ is hiding some stolen cocaine.
My Deepest Secret: Midway through calling 911 when he sees that Sophie is still alive, Yohan realizes that since he found her in time, the attack hasn't happened all that long ago. Meaning that the killer is still in the building, and furthermore Emma could be in danger.
When Roy and Xykon battle for the first time, Xykon gives Roy an Oh Crap moment that triggers a brief Heroic BSoD. When the Heroic BSoD ends, it's Xykon's turn.
Even within the limits of being a skeleton drawn with stick-figure art, Xykon's expression when he's confronted by the ghost-martyrs of the Sapphire Guard, led by Lord Soon himself in this strip conveys this reaction loud and clear. Later in the same battle, it happens again, after Soon reveals that he knows exactly how to permanently kill a lich. And then Soon has one when he realizes Miko's about to destroy the crystal.
Belkar Bitterleaf, though a hero (arguably), suffers one in #568. After finally getting fed up with an annoying oracle, Belkar kills him. Unfortunately, he also has a Greater Mark of Justice curse on his head, set to go off when he kills a person within the sphere of influence of a town. And guess what popped up around the oracle's home last week? The name of the town manages to make it even more hilarious.
Belkar: I don't get it. What does that... [Mark of Justice appears on his head] Belkar ...mean? [Mark of Justice goes off] Belkar: AAAAHHHHH!
Elan, Therkla, and Qarr share one in the second-to-last panel of this strip.
Therkla: Did that tree... Qarr: ...just say "Oww"?
Xykon again when his phylactery is about to fall into the Snarl's prison. He has a brief moment of relief when it doesn't, only to get the look again when it instead falls into a sewer with pipes leading to a sewage plant, a labyrinth, and the ocean.
Admit it. You were thinking it about halfway through this page.
V's expression in the penultimate panel of this comic speaks volumes.
Durkon gets this look here when Malack reveals that he snuck a code word into Mass Death Ward that lets him dismiss it.
Malack gets one once Zz'dtri removes his Protection from Daylight spell.
"Hello, Nale. Walk with me. We have a lot to talk about."
At the end of this strip, there's a splash panel showing us both Roy's "Oh Crap!" expression, and why he's right to have it. (Belkar still hasn't worked it out.)
Randall gets wide-eyed and starts trembling when he realizes that the kitten he drove away with threats in the past now has a very protective canine big brother.
In a Halloween strip, Brutus, trying to take a nap, only half-pays attention as Pixie tries to get him to come see a movie she found on TV, Nighttime on Elmo Street. Then she says that one of the guys in the movie is called "Freddy Krueger". Brutus' eyes snap wide open and he takes immediate action to protect her innocence.
Pixie didn't know she had claws. Brutus told her she had them, and how to extend them. "Just flex your fingers." She did, and was shocked to see them appear. The "Oh, crap" is unstated in the comic, and left as an exercise for the reader. (which maybe makes this an aversion, but how many readers do you think failed to say it?)
A hawk's facial expression shifts in the space of one panel from quietly menacing to alarmed when it tries to attack Pixie, only to find itself snared in Brutus' jaws mid-swoop.
In Pokemon Emerald Double Nuzlocke, Crys, who is the Champion of Johto since she beat Gold in the final battle back in the Crystal Double Nuzlocke, protects Sapphire, and Archie taunts her that she cannot scare him, but Crys says that she has old folks, which is the team Crys and Gold respectively used. Archie's reaction alone sells this.
Archie: What the actual fuck!! That bitch has a fucking army!
Sees a unique use in thisSaturday Morning Breakfast Cereal: A man says he wants a photo of himself etched to his tombstone that looks like him when he died (as in not younger), and then gets stampeded by a herd of cows while driving, leading to his "oh crap" face being immortalised.
Although the tension is deflated a moment later, Kevyn suffers this in Book 6, Part IV, after Ennesby gives a damage report.
Ennesby: Low-yield kinetic strike to the fore dorsal turret ... Hull-breach above the wardroom! Kevyn: Wait, dorsal turret access is right up there. We're in the wardroom.
And in this one. Schlock is talking to some rogue nanites that have infiltrated his eyes and hijacked them in order to create a simulated space to speak with him. Combat between members of his species half consists of using their extremely potent and adaptable immune systems to counteract the various chemical cocktails their opponents synthesize in an attempt to melt and absorb them. And he's just revealed that the plasma cannon he's been "impotently" waving at the nanites was actually just a familiar visual metaphor for his immune system. They have very good reason to look so worried.
Rule 3: An ordnance technician at a dead run outranks everybody.
A twofer in the last row of this comic. Even if it's just a still picture, the last target's eye view of an oncoming missile is never happy.
A particularly nightmarish revelation left the Doctor, Tagon, Schlock, and Ennesby all similarly horrified.
"I'm ready to start burning the corpses."
One that's both good and nightmarish: Kevyn is outlining his theory that the Pa'anuri are taking long-gun potshots at the Galaxy's bigger annie plants in an effort to "walk their shots" and finally nail their nemesis Petey... and learns, by way of witnessing an entire city explode in front of his eyes, that they're getting dangerously close. Petey is less surprised, but isn't happy either.
"Tell me more about this timing thing...."
Todd from Selkie in this comic when he begins to realize that all is not going well between the titular character and Amanda, her arch-antagonist and recently-discovered adoptive half-sister.
Shortpacked! features an immensely satisfying one - Head Alien II has been kicking the heroes around while giving them a Hannibal Lecture, but just as he tries to crush Leslie to death, Robin arrives.
Sluggy Freelance (sorted by chapter or story or theme as appropriate):
In "Vampires":
Valerie: "I told you to stay out of my business, you bitch! Once you're an immortal I will take great lengths to see you suffer in ways impossible for a mortal to understand. Don't think Sam can protect you, he is far too weak!
Zoë (thinking): Wow! Vampire PMS!
Valerie: "We can read thoughts, you know."
Zoë (thinking): Oh crap!
Valerie: "That is an understatement."
"Mecha Easter Bunny" is practically built on a long sequence of these.
Bun-bun when he torches the Mecha Easter Bunny and finds that he's only revealed an unharmed, heavily armed robot.
When Bun-bun pulls a bazooka on the Mecha Easter Bunny and right at its face, it tries to frantically scramble backwards, and its "internal robot view feed" also shows the text ">OH CRAP!!!!"
Mind you, a bazooka shot to the face barely scratches the MEB, leaving Bun-bun to have another "oh crap" moment.
Santa Claus when he realises the MEB, which he built, is about to kill innocent people. "Ho-ho-holy $#!%!"
Santa again when Bun-bun manages to destroy the MEB, and Santa knows he'll be coming after him next. "Ho-oh crap."
Finally, Bun-bun when he thinks he's decapitated Santa and it turns out it's a dummy full of explosives just about to go off.
In "Dangerous Days Ahead": When Hereti-Corp troops led by Mr. Killum disable Riff's robot with an EMP, only to find that inside is actually Oasis, a One-Woman Army brainwashed to kill all Hereti-Corp personnel on sight. (Killum: "Oh shi*...")
Father Time:Overlord Bun-bun has arrived on the wings of the Groundhog's Shadow, and now faces the one who stole the Deus Egg. The Year 2003 himself!
2003: "I have done everything I can to thwart you peacefully, Bun-bun, but I've trained half my life for this fight. You came for the egg, I'll leave with your head!"
Bun-bun's multi-part switchblade scythe of doom:KA-CLIKITY CLICK KLAK
At a meeting of the demons where Psyk tries to talk about the Canada situation but Lord Horribus isn't paying attention, Psyk snaps to him about how he's obsessed with finding Torg. The strip ends with Psyk and all the other demons present staring with the realisation of his having gone too far. (The next one starts with the same picture: "Still going 'oops!'")
"Mospinispinosp" is a demon consisting of a snakey humanoid female body (Mosp) with two small talkative snakes (Isp and Osp) for "hair". At one point, Isp and Osp wait for Mosp to apparently be asleep and confide to another demon that she's been helping mortals and betraying the demons. They certainly have an "oh crap" moment when they realise the "demon" was just a sock puppet on Mosp's tail, and when they see the diagram of what she's going to do to them now (which is to make "earrings" to clip their mouths shut).
Later Isp and Osp again when they think Mosp has killed their Demon Lord — and even more so when they find out she failed. "Holy demon-crap!"
"A Time for Healing": Jane, an intelligent zombie who's giving Gwynn a "The Reason You Suck" Speech while preparing to eat her brain and mocking her claims of a Tome of Eldritch Lore that gives her magical powers — when she pushes her Berserk Button and finds out that that's what activates said powers, and they're way out of her league. Her face is not shown at the Oh Crap moment; it's instead indicated effectively by her surgical saw winding down.
"4U City Red — Another Year in the Life of a Villain": An ordinary, non-talking frog (usual line: "CROAK.") is caught in a building after a somewhat literal-minded One-Woman Army has been ordered to kill everyone in it.
"CRapK."
"The Heavens and the Earth": Downplayed: A character who doesn't really die when killed notices she's about to have her throat cut. She only looks annoyed as she goes "Oh crap." In fact, a couple of other characters have a much bigger "Oh crap!" reaction to the same thing because they anticipate that slicing someone's carotid artery in a zero-G environment is going to end up with lots of blood everywhere.
A Running Gag where Dr. Schlock thinks he's just got out of trouble and then Riff appears and sticks a laser cannon to his head.
Tenko from So... You're a Cartoonist? has one here when he realizes Curmudgeonly Carl will be reviewing his portfolio.
Stand Still, Stay Silent. Between the crew ending up with stowaway (who himself thought he was smuggling himself to a place that is not a dangerous Forbidden Zone), close calls while fighting monsters happening once in a while and some characters getting early warnings for bad things that will inevitably happen, there are plenty of opportunities for this.
In Starslip, Vanderbeam, understandably, has this on his face for four panels in a row after the Anthelerix relocate every known celestial body and planet to another part of the Milky Way.
Subnormality: A newly-arrived Hell-damned soul's smugness gets swept away as he realizes that he's about to receive a horrible inescapable beating from a large and violent man; not just his face shows this, as he's also wearing a shirt which displays his thoughts: OH SHI-
Tiger, Tiger: Honeyfoot's eyes go wide when he's rowing back to the Gallant with a hungover Jamis and recognizes a ship at port that should be on the other side of the ocean. It appears that taking a major merchant ship for a joyride caught up to him much sooner than he was expecting, and he hurriedly tries to make Jamis look presentable as he panics.
In Two Evil Scientists, a robotic clone of Dr. Wily uses Evil Energy to revive Bass at the insistence of his daughter Cream. It works, and he has a few moments to enjoy his success until he realizes he brought Bass back to life, and he Came Back Wrong.
unOrdinary: Isen has himself some panic when he realizes that John was expelled from his previous school not because he beat up half his class, but because his previous school couldn't control him and he's incredibly powerful. Not helped by the fact that Arlo is so obsessed with the hierarchy of power he won't leave him alone and is in the process of forcing him to reveal his true power.
Turas enters a room looking for Sette and spins around when the door is slammed behind him. He has just enough time to spot Duane — who he knows to be a powerful wright — removing his hood to reveal his face and let Turas make the horrified realization he's in for a very messy death before the scene cuts to a soundproofing pymaric activating outside the door.
During the attack on Litriya Shrine Elka looks out the window just in time to see that their oppenents are carrying an allepakh, a Fantastic Nuke that heats up everything in a square quarter mile to its melting point on detonation, not to mention the damage caused by the resulting fires. Her reaction is a wide eyed "Oh shit".
Ghost of Wake of the Clash pairs this with one of the few glimpses into a character's inner monologue. We find that he’s not the brightest when it come to staying out of trouble.
After saying that he's sick of his work and sick of Donald Na, a distinctively tattooed hand reaches out from the darkness and taps Wolf on his shoulder. Wolf's wide-eyed fear says it all.
In their first fight, Donald offers Ben a free hit. He takes it and Donald reflects it with such force that it immediately throws Ben into a panic.
Jaeryeong is initially dismissive when some Cheongang mooks report that they can't get in touch with the rest of the gang. He tries to call up Giju, one of the leaders, only for the phone to ring and ring... and then he remembers with sudden dread the two rival gang members who had beaten the crap out of him the other day...
Jack, when Crystal points out to him that Sandra may not wait until nightfall to come at their home, and completely falls into panic mode once he hears her scream. Sam is unfazed.
Sandra has a big one when Viv makes her realize that without her "sticking around", she probably would face a little problem:
Viv: Well, try this. Now we're surrounded by feral gremlins. The fire I made will keep them away until dawn. I know this stuff. If not for me, you'd be eaten. Sleep on that.
Lord Incubus himself has one when Mabel appears out of nowhere and completely sabotages his attempt to corrupt Sandra.
When Jake realizes that Sandra is back from her banishment, he immediately panics. The other members of the group are not particularly happy either, with Crystal being the sole exception.
Crystal went to her hometown alone in order to find Sandra. Instead, vampires find her, with one claiming they have instructions to kill her. Cue panic.
The fat lady vampire who tried to kill Crystal had this reaction when she saw a very big and angry Wally in his werewolf form roaring at her.
, when the ultimate Jerkass discovers he's finally bitten off more than he can chew.