In Animorphs, the final book has two great ones in a scene. The first is Visser One, when he realizes he's lost. He doesn't say anything, he just slumps, and for the first time in the series, isn't angry. The second is Tom, when he realizes that Jake is alive.
Probably The One, upon hearing: "Ram the Blade Ship".
The Keepers in House of the Scorpion describe their side of the story to Esperanza of Matt and the Lost Boys' rebellion at the colony where they are held. Esperanza seems to believe it, right up until she asks about the drugs the Keepers have been taking. When they deny that they have been, Esperanza promptly submits them to a drug test, and they are led away by the authorities.
While we can't see it, the description of Be'lal's death in the climax of The Dragon Reborn leaves very little doubt that he had this kind of reaction. Understandable, since The Chessmaster was moments away from the fulfillment of his Evil Plan before being blindsided by someone he thought he had removed from the equation and promptly rendered Deader than Dead.
Asmodean's death was much the same way, if his last words are any indication ("You?! NO!").
When Semirhage enacts her plot to capture Rand using the male A'dam, Rand gets a huge one. Quickly followed by Semirhage getting her own Oh Crap moment when Rand uses the power of the Dark One.
Rick Riordan's The Heroes of Olympus has one for Alcyoneus after Frank and Hazel drag him into Canada. Also, Polybotes when he realizes that Percy’s got a god on his side.
Harry Turtledove's Worldwar series is basically one big Oh Crap moment for The Race. Oh, it's only been 800 years, how far could the savages have gone since 1142? Wait, is the planet emitting radio waves?
Plus, every Lizard viewpoint character has at least one of these.
Three such characters, two shiplords and an artillery supervisor, are used for the sole purpose of screaming OH CRAP before the Germans or the Americans kill them.
The career of Flight Captain Teerts is a series of these events. First, his jet is shot down behind Japanese lines due to the world's luckiest bullet hitting his engine. Not knowing how the Japanese treat their prisoners, he surrenders. He's hauled over to Tokyo and interrogated on how to build a nuke. He tries to confuse the Japanese scientists by telling them things like "Nukes use hydrogen and plutonium", only to discover that they already knew about fusion reactions. He is eventually rescued, only to nearly die when America nukes Miami, finally dying when caught in a similar blast outside Denver.
The conclusion of the final book, Homeward Bound, is literally one big Oh Crap moment for the entire Race.
Cersei Lannister's response to disagreement from the new High Septon in A Feast For Crows.
And, in A Storm of Swords, the end of the duel between the Red Viper and the Mountain.
Viper: Say it! (stabs Mountain) Mountain:(grabs Viper and slowly crushes his windpipe) Elia of Dorne! I killed her screaming whelp! Then I raped her! Then I smashed her fucking head in! Like this! Everyone: Oh shit!
The most epic/tragic one is provided by Catelyn Stark in A Storm of Swords at the Red Wedding, when she grabs Edwyn Frey's arm and feels the armour beneath his sleeve, thus realizing exactly what is about to happen.
In the fifth book, this is Quentyn Martell's reaction when he realizes he's on fire.
Oh, he thought. Then he began to scream.
From the same book, Janos Slynt when his smugass defiance forces Lord Commander Jon Snow to order his hanging. The smugness returns when Jon decides not to hang him, only to vanish again when he learns that this is because Jon feels his should behead him, Stark-style.
In the prologue of A Storm of Swords, Chett has this, too. At the first horn blast, he has a minor one and is infuriated that his plan is going awry. At the second, he gives up the plan and gets ready to fight. And then...
Sam: Three, that was three, I heard three. They never blow three. Not for hundreds and thousands of years. Three means—
In A Dance with Dragons, both Kevan and Jaime Lannister feel dread when they see the white raven from the Citadel that marks the beginning of the long winter. After the disastrous civil war, Westeros doesn't have nearly enough food to endure decades of winter.
Ned recalls that the moment Rhaegar Targaryen passed over his own wife to name Lyanna the Queen of Love and Beauty after winning the Tourney at Harrenhal "all the smiles died". Everyone present knew that wasn't going to end well.
The Vorkosigan Saga likes this. In "A Civil Campaign," Richars Vorruyter is preparing to taunt Miles as Count Vorhalas and the other conservatives arrive to give him a majority of votes, when Vorhalas tells Richars that he knows what a douche he's been and is thus not going to vote for him. Later in the same scene, Richars tries to accuse Miles of shenanigans with Ekaterin, who proceeds to stand up from the balcony and propose to Miles in one of her Crowning Moments of Awesome.
For that matter, Miles causes another one of these in The Warrior's Apprentice, where he arrives just in time to testify on his own behalf in a treason trial, and splits up the pair of villainous ringleaders by outlining how one is planning to betray the other. The first gets an Oh Crap moment as the second begins spilling the beans.
This being Discworld, he got a chance to finish it, as his ride to the afterlife shows up. "—t."
Later in the series, Greebo the cat invokes an identical half-line of dialogue — the latter when he finishes becoming human, the former when he starts reverting to a cat — when he suffers an involuntary shapechange.
"OHSHITOHSHITOHSHIT I'M GONNA DIE!" is practically Rincewind's catchphrase.
To the point that when Butterfly asks him to say "something in wizard language," his first response is "Stercus, stercus, stercus, morituri sum."
Trev Likely has an internal-monologue Oh Crap moment in Unseen Academicals, when he realizes that the wizard who's supposed to use magic to prevent any cheating on the soccer field has just blocked all magic, including his own, from operating there for the duration of the game.
It is frequent to hear OH SHIT whenever anyone realizes they've just said Monkey around the Librarian.
From Small Gods:
"Those watching said, later, that when Vorbis looked up, his expression just had time to change before two pounds of tortoise, traveling at 20 meters per second, hit him directly between the eyes. It was a revelation."
Likely the response of New!Death to this:
But Bill Door was already rising and unfolding like the wrath of kings. He reached behind him, growling, living on loaned time, and his hands closed around the harvest scythe. The crowned Death saw it coming and raised its own weapon but there was very possibly nothing in the world that would stop the worn blade as it snarled through the air, rage and vengeance giving it an edge beyond any definition of sharpness. It passed through the metal without slowing.
Bill Door: NO CROWN. NO CROWN. ONLY THE HARVEST.
In David Eddings' Enchanter's End Game (5th book of the Belgariad), one can easily imagine Zedar's face when Belgarath re-enters the room after Zedar kills Durnik in Torak's tower, performing the one act that left him with no chance of redemption and a one way ticket to And I Must Scream territory, being buried in the earth alive.
Earlier in Magician's Gambit Ctuchik, in a moment of desperation, tried to use sorcery to destroy the Orb with the command "Be not!" His terror caused him to forget that was one thing the universe itself would not allow, and he had a single moment to remember before he was not.
An even older example occurs in the 18th book of The Odyssey: Odysseus, posing as an old beggar, is challenged to a fist-fight by an actual beggar, Iros, who is big, but neither strong for his size nor really a fighter. After vainly trying to dissuade Iros from this, Odysseus finally agrees to the fight and girds his loins for the fight. And then the watching suitors and Iros see the muscles on Odysseus' legs, chest and arms...
Honor Harrington has several, often when some new wrinkle in technology is unleashed. One of the best is from minor recurring character Genevieve Chin, who survived all the way to the final battle in the eleventh book, At All Costs. Her enemies fire multiple-stage missiles that proceed under power for some time, coast ballistically through space for a long time, then fire the second stage and do attack runs. Her initial skepticism — that these missiles would be easily stopped because the range would prevent their parent ships from relaying targeting data — was quickly overruled when she remembered that her opponents had recently unveiled a new technology allowing them to control the missiles in real-time from virtually any range. Unfortunately, she was two minutes too late, and Chin and her entire fleet were wiped out in what was probably the biggest massacre of the series up until that point.
The opening of that battle also contained an Oh Crap moment for Honor herself as she realizes that yes, her home IS being invaded by a fleet that is numerically larger than any fleet fielded in recorded history.
The probable reaction of the Havenite task force unlucky enough to be in the way of the Grayson Space Navy after Honor was apparently executed, as Fleet Admiral Judah Yanakov was a bit displeased by this turn of events. As was every soldier under his command. His order? "Lady Harrington, and no mercy!"
That command inspired an Oh Crap from Admiral White Haven as well, especially once the crippled Havenite ships began to launch escape pods, because he thought he was about to witness a massivewar crime, until he realized that the order was No Mercy,note not picking up survivors as opposed to No Quarter.note ensuring that there will be none
The reaction of the leading bureaucrats of the Solarian League when they are told that Admiral Sigbee surrendered her data cores to Mike Henke intact.
Pavel Young has a rather impressive one in Field of Dishonor when he watches Honor utterly annihilate the professional duelist he'd hired to kill her, followed by Honor's declaration on live HD that he was next.
Mission of Honor has the SLN task force under Admiral Sandra Crandall getting a taste of Manticoran missile spam, as well as the Manticorans reacting to Operation Oyster Bay.
The very first book, On Basilisk Station, has a very strongly implied one on the part of the RMN's Home Fleet, after Honor signalled "Case Zulu." In RMN communications practice, Case Zulu always is taken to mean Invasion Imminent.
This is the near-verbatim response of the Big Bad in charge of the Mesan Alignment note Exact words: "oh, shit!" in A Rising Thunder on learning that Manticore-Haven diplomatic relations have surpassed Mesa's worst-case scenario. As in, they're military allies. And it goes From Bad to Worse: the secrets of Mesa's ultra-fast streak drive and ultra-stealthy spider drive? Blown. The secret of Mesa's viral-nanotech assassination technique? Blown. The secret ultimate identity and strategic goal of the Mesan Alignment? Blown. And it only gets better from there...
There's an interesting mix of Oh Crap and lack of Oh Crap moments as the conflict between Manticore and the Solarian League builds in Mission of Honor: various Solarian commanders have Oh Crap moments as they find themselves on the wrong end of increasingly lopsided battles (a group of cruisers and destroyers defeat seven battlecruisers, six battlecruisers defeat twenty battlecruisers, seven heavy cruisers defeat 71 superdreadnaughts), but the Solarian high command remains assured that once they concentrate a sufficient force, Manticore will be crushed like a bug.
Flag In Exile has a good one, when the Havenite fleet attacking Yeltsin, composed mostly of battleships, notices that the Grayson battlecruisers have just assumed a formation that is insane for battlecruisers... but ideal for superdreadnoughts.
A Rising Thunder has one on the part of Admiral Filareta, who is up until that point convinced that the comparatively tiny Manticoran fleet couldn't possibly touch his fleet of over three hundred ships of the wall. Then Thomas Theisman appeared on Honor's flag deck. And he was there voluntarily. When Filareta realized what this meant — that Manticore is now allied with the Republic of Haven — he most certainly had one of these. Yes, the two best active fleet commanders in the galaxy were sharing a flag deck. Whoops.
The climax of Good Omens. "For the second time in 6,000 years, Aziraphale swore."
In The League of Peoples Verse novels by James Alan Gardner, the explorer corps (aka "the expendables") are so used to experiencing such moments that "going 'oh shit!'" is a euphemism for dying.
Dragonlance: Raistlin Majere, upon seeing Tasslehoff (who, as a kender, can screw up the timestream) in the past, reflects on what this means for his plan to ascend to godhood. Precisely three thoughts later, the reflection is "I might die!"
In Island in the Sea of Time, the character Swindapa is captured, brutally raped, and given nasty STDs by her culture's enemies, then given as a present to the Nantucketer admiral, who is an utter badass black lesbian coast guard officer and martial artist, who frees her and teaches her how to use a katana. In the climactic battle, Swindapa ambushes her erstwhile tormentors, who barely have the time for an Oh Crap moment before they're beheaded/disemboweled. In the same battle, the Big Bad William Walker has one when the Nantucketers manage to blow up his supply of gunpowder, which fueled the only 3 cannons in the world which had been busily enfilading the Nantucket lines.
In the book Fool, whenever the fool's plans are derailed (about once a chapter, till in the end when his whole plan pays off. IN SPADES!) or on the last page when the witches show up to claim their price, which turns out to be Spain, his automatic response is a simple "Fuckstockings."
Happens to Freddy twice in the book A Nightmare on Elm Street: The Dream Dealers. The first time, Freddy wakes up after being briefly knocked out and spots a bomb (which kept showing up in dreams) lying next to him, seconds before it explodes. Later on, when Freddy pulls himself back together, he finds himself confronted by the escaped souls of his recent victims, who decided to pay him a little visit before crossing over to the great beyond. Freddy is pretty much left speechless:
A rare sighting of the invoked "Oh Crap" in Isaac Asimov's Second Foundation. You really don't want to read this if you have not read the book, though. Really. When the Mule realizes what the plan of the Second Foundation is, and how he has been duped, he despairs for a brief moment. This is exactly what the First Speaker was counting on, and during that fraction of a second he went into the Mule's mind and destroyed his hostility towards the Second Foundation and ambition to conquer the galaxy, as well as erasing all memory of the encounter.
Opal from Artemis Fowl: The Opal Deception. Her master plan, like many, involves a massive bomb. Suddenly, her thugs notice the bomb has gone missing, and receives a taunt from the heroes, who have just snuck into, and out of, her shuttle. However, she still has the detonator and the opposing shuttle is in the right place, so she presses it. Only then does she realize they didn't steal the bomb, just hid it.
John Taylor, protagonist of the Nightside series, has cultivated a reputation so nasty that he can often get an Oh Crap reaction from the opposition, simply by introducing himself. A pair of huge demons summoned to attack him once took one look at him, went through simultaneous Oh Crap revelations, and turned round to berate their summoner for pitting them against him.
Bit of a subversion, as Taylor is often bluffing.
or is he?
During the duel between Westley and Inigo in The Princess Bride, this is what you think is going through the mind of Westley after Inigo reveals that he's not left handed, but then Westley gets him right back and reveals that he isn't left handed either! Inigo's reaction is priceless.
Subverted in the battle of wits against Vizzini, when Vizzini doesn't even have time to make the "Oh, Crap." face after he realizes that both cups were poisoned.
In Spock's World, McCoy gets one when he scans the names of secessionists and finds one he knows... T'Pring.
In Perdido Street Station, the managers of Cadnebar's, an illegal gladitorial arena, play up the drama for the crowds by deliberately setting up Oh Crap moments for naive yokels, who sign up for what they expect will be an easy two-against-one fight. Too bad they don't know it's two of them against one weaponized killer Remade or professional cactus-man mercenary until their opponent steps onto the field....
Also, Simon/Silas gets a doozy of an Oh Crap in The Scar, when he realizes he'd neglected to mention the hundreds of little tugboats that used to drag the floating city of Armada in his report, meaning the Crobuzoner navy's about to get its ass handed to it by a mass kamikaze assault of unmanned floating bombs.
Jim Butcher must love this trope. Practically every single one of his books (bothseries) features one of these, usually as the main characters realize that the situation, already sucky, just went from worse to "OMFG".
Done at least once per book in The Dresden Files. Almost each instance requires multiple books of Infodump and backstory, plus a few Noodle Incidents thrown in for flavor, to really understand the full power and scope of exactly how screwed Harry is, though.
In Turn Coat, Harry manages to induce this in his mentor after casually mentioning that there were more parties about to show up. Only the mentor realized what Harry's planreally was. McCoy takes it fairly well, all things considered.
Harry: (Grinning like a maniac) Wile E. Coyote. Suuuuuper genius.
In Changes, the thing that lets Harry know just how bad things are is when he talks to Mac, the usually monosyllabic bartender, at the beginning of the book. Harry's reaction to Mac speaking for a good paragraph is this:
I looked at him, shocked. He'd used... grammar.
Changes also gets a good one in early, as a Red Court Vampire who's part of a hit crew runs into Harry... and runs screaming in the other direction.
In Small Favor Nicodemus gets one when Harry starts strangling him with the Iscariot's Noose (the Noose will protect its wearer from anything except itself) and he realizes that he might actually die.
In Codex Alera, this is usually caused by an enormous army appearing out of nowhere. The Vord in particular are so good at causing this that even a fairly small group turned large portions of the second book into a string of such moments as Bernard and company fall into trap after trap.
In 1984, this is pretty much how Winston and Julia react when the telescreen that was concealed in their hiding place the whole time makes itself known — moments before the Thought Police show up and arrest them both.
Don't forget Harry's reaction in ''Deathly Hallows, when he looks at Snape's memories. He's just been overwhelmed by images of his mother, revelation's of Snape's character, guilt that he left him to die, not knowing the truth, and now - Harry has to die, leaving everyone behind, hoping someone will finish Voldemort off. This is a Tear Jerker, Crowning Moment Of Awesome, Crowning Moment of Heartwarming AND an Oh Crap moment. That in itself is epic.
Also Voldemort's reaction after Neville kills Nagini.
At the end of Michael Crichton's Timeline, the villain is sent back in time, with no possible method of return, to 1348 Europe, the year when the Black Plague (which can be spread through air) had killed one third of the population of the place he ended up in. He goes into full on Oh Crap mode when he realizes this, and then he begins to cough...
While we don't get to see his reaction in the films, in Death Star Tarkin has enough time to shout "Unthinkable! Unthinkable!" as his precious Death Star explodes.
At the end of The Last Battle the Calomene Warlord Rishda gets a huge one when he discovers that the demon Tash is very real and very angry with him.
In one of Creator/Roger Zelazny's The Chronicles of Amber series, a traveler encounters a demon guarding a bridge. The demon draws a (literal) line in the sand, and makes a Badass Boast about the consequence of crossing it. The traveler points out that there is one specific kind of being who could easily defeat the demon. The demon admits this is true, but thinks the encounter highly unlikely. The traveler then steps across the line...
Second Stage Lensmen: Kim Kinnison has been playing a long game, working his undercover way right up to the top of the Thralian dictatorship. When he comes back home after a battle, with a Patrol fleet disguised as the Boskonian one he's just annihilated, his twelve Co-Dragons are summoned to meet him. Cue the Oh Crap when not only are they disarmed, but the man they thought was the Capo di Tutti Capi among the Boskonians climbs out of his armour with a Lens on his wrist.
From Under the Dome: Rusty has this reaction when he realizes that Jim Rennie's several-hundred-gallon propane supply is about to explode and flash-fry the enclosed town.
In The Stand Flagg and his followers all freak out when they see what Trashcan Man brought back with him from the desert: an armed nuclear warhead.
In The Dark Tower's final novel The Dark Tower, Walter (a.k.a. the Man in Black) has one when he realizes his thought cap was NOT preventing Mordred from reading his thoughts.
Also in The Dark Tower, Mordred has one when he realizes he ate very rotten horse meat which is sickening him and might eventually kill him.
As the Great Lakes Storm of 1913 intensifies, Captain McTavish and his wheelsman look on in horror as another lake freighter swings too close to the SS Longhope— they avert the impending collision just in time to see the monstrous waves tear the other boat to pieces and drag it to the bottom of Lake Huron. A similar fate awaits them when their boat's steel plates give way.
Still believing all will end well, Clara Grace follows the First Mate to the deck... only to discover what's left of the crew swinging out the last remaining lifeboat which is ripped from its winches and hurled into the roiling lake.
Having survived the wreck of the SS Longhope but likely dying of exposure, Clara Grace must watch as bits of wrecked ships and the dead, frozen bodies of her friends are violently slammed ashore.
There's a great one in Excession when the ship that's been following the Sleeper Service realizes that it's converted all its internal volume into engines.
The Yawning Angel: Two hundred and thirty-two thousand times the speed of light. Dear holy fucking shit.
In Bloodstone, part of the Stones of Power series by David Gemmell, the Big Bad realizes that the legendary "Sword of God" is a nuke, and he's been transported to the site of the first testing of one. We get the immortal line "On wings of fear Sarento ran." He is annihilated soon after.
All the way back in The Bible. Specifically, the Book of Esther. More specifically, when Haman realizes he's inadvertently ordered the death of the Queen, whom her husband the King values enough to have been willing to offer up to half his own kingdom at her request.
In the Sword of the Stars novel The Deacon's Tale by Arinn Dembo (one of the game's writers), the protagonist Cai Rui is investigating a colony destroyed by the so-called Rippers (AKA Black 13). Then his assistant finds the last message sent from the colony, which never arrived. It was sent over a year ago. Except the distress signal arrived only last week. Cai turns on his motion detector and finds hundreds of signals rapidly closing in. He's the only one who makes it out alive.
The Robert E. Howard short story "The Extermination of Yellow Donory" is a series of Oh Crap moments. The titular character, sick of being pushed around for being a coward, decides to commit "suicide by gunslinger" by seeking out and loudly berating a well-known, greatly feared gunslick. However, his plans go awry; the gunslinger's utterly spooked by this apparition, and thinks it's some sort of a trap for him, so he runs out of the tavern and high-tails it out of town...and "Yellow" Donory looks around to find that now everybody in that tavern is scared of him.
Happens with varying degrees of comedy and drama in the Aunt Dimity series. Notable examples include:
In Aunt Dimity Detective, when Lori learns the police have Kit pegged as a likely suspect in Prunella Hooper's murder and are questioning him about it. Gossip ceases to be a jolly pastime when it might land a good friend in jail.
Likely the biggest one to date is the moment at the end of Aunt Dimity and the Deep Blue Sea when Lori returns to her suite in Dundrillin Castle after she and Damian Hunter get the relatively innocuous explanation of the island's prosperity from Sir Percy and the island's elders. She sees the mirror door to the emergency stairs standing open (sans alarm) with Reginald and a toy soldier on the floor near the doorway. Then she comes into the room, sees the footprints in the dust and hears her son Will cry out "Mummy!" Realizing the crazy stalker has her sons, she follows him down the stairs and out into a Force 9 gale.
Played for Laughs in Aunt Dimity Down Under when Lori experiences her first earthquake while she and Cameron are visiting an art gallery in search of Bree Pym. At first, she doesn't understand what's happening, then after she's told, she can't understand why the locals are so calm as they wait it out under the furniture.
Also Played for Laughs when Kit extracts his condition from Lori in Aunt Dimity: Vampire Hunter: Lori must promise to take riding lessons despite being afraid of horses.
In the illustrated book I Want My Hat Backthe rabbit is clearly scared when the bear is running at him screaming: "YOU! YOU STOLE MY HAT!" He is promptly eaten.
In the Paladin of Shadows book Unto the Breach, Mike has one when he learns that there are 4000 Chechens after his ~100.
In one of the stories from The Sandman Book of Dreams, Paramore a magician has been abusing his powers and divulging rumors of Morpheus, when he finally goes to sleep he finds someone and tells him a story of how he managed to unbestknowlingly get offend a great prince, cue Dream quoting the rumor.
At the climax of Gardens of the Moon, the first book in the Malazan Book of the Fallen series, the Bridgeburners suddenly realise that burrowing cussers—satchel charges—under the streets may be an excellent way to sow terror and soften up the city for invasion, but there won't be a city left after the detonation when the city is built on a pocket of natural gas.
Midas from Delirium also has one after a family memeber of Pablo Escobar's (yes, the drug dealer) tells him the plan that Escobar laid for negating her to enter to his gym.