David from Animorphs after he is trapped as a rat.
In Dr. Greta Helsing 3rd book Grave Importance, the end of the world has happened and only a handful of humanity is surviving. A broken-hearted Varney the Vampire prays for the first time in centuries and asks God if this is what he wants. And for the first time in 80 years, God answers back and let's out such a huge "Noooooooooooo!" that the font of the Big "NO!" takes up 1/3 of the page in the book. Godthensaves the world.
An unintentionally hilarious example is seen at the end of Marianne Curley's The Key, where Ethan screams it after his true love Rochelle takes a magic arrow to the chest for him. Rochelle, who is narrating the story and using present tense, is able to tell us that the last thing she hears before she dies is Ethan screaming "Nooooo!"
In the first Wild Cards book (from 1986), Dr. Tachyon shrieks "Noooooooooooooooooo!" when he believes he has failed to prevent his friend Angelface from being shot.
In George R. R. Martin's A Clash of Kings ship-captain Davos Seaworth unleashes one of these during the bloody naval assault on King's Landing, when he realizes that one of the other attacking ships is about to set off a particularly nasty trap.
The Gaunt's Ghosts novel The Guns of Tanith uses this at the climax when Mkoll kills Sagittar Slaith.
A common dialogue in the Goosebumps series. Whenever the protagonist or any other character in each story finds his/herself in deeper shit, this is the dialogue that will predictably be uttered, usually out of horror.
Ron gets another one in Deathly Hallows after he hears Bellatrix give Greyback permission to kill Hermione.
Starcraft: Ghost: Nova. The title character does one of these every other page, just about. The other pages, somebody else fills in for her.
In Magician, Pug is confronted with a stadium filled with people from his home country, taken as slaves (just as he once was). When the slaves are commanded to fight each other for the amusement of the crowd, and they refuse, Pug empathizes with them and flips his nut when the warlord tries to have the slaves killed for their impudence. Being a massively powerful magician, Pug basically pulls the stadium asunder in his rage. More of a metaphorical "no" than a literal one.
In Warrior Cats, Bluestar (then Bluepaw) bursts into one of these when her mother is killed in battle, complete with eight O's.
In The Hobbit, Gandalf tells Bilbo about Beorn and says that he is a 'skin-changer.' Bilbo expresses his thoughts by asking whether he meant 'furrier', to which Gandalf responds: 'Good gracious heavens, no, no, NO, NO!'
" Rikash - no! someone cried in a voice that cracked as it rose. "No! No! NOOOOO!" It was her voice. If she screamed loud enough, long enough, he would live. She hadn't realized that he meant something to her. She hadn't realized he was her friend."
Fandomar gets one with multiple "N"s in Galaxy of Fear, when she sees that The Virus has escaped. Though she doesn't tell anyone that that's what the empty room means.
Defied by Ciaphas Cain, THE HERO OF THE IMPERIUM, in Caves of Ice, at least according to his narration, when he's asked about whether his group should examine a tunnel probably leading to some Necrons: "'No.' It took all the self-control I could muster to sound calm and collected, instead of screaming the word."
After Mr. Click's leg comes off in A Bad Day For Voodoo, Tyler and Adam go to Tylers house to watch the news, and the reporter announces that Mr. Click is dead. Adam shouts, "No!".
Many of the Pigeon books for children have the reader do this, usually in response to the Pigeon asking to do something reckless or improper. The Pigeon himself delivers one in "The Ducking Gets a Cookie?"
Journey to Chaos: Tiza releases a short and sharp "No!" when she realizes she's about to relive the even that lead to her amnesia by the same guy who kidnapped her when she was younger.
In the sci-fi novel Limbo by Bernard Wolfe, the protagonist gives a NO that takes up an entire page. Towards the end of the novel he gives a Big "YES!" in the same way.
In Titus Crow, Cthulhu...yup that Cthulhu, can only scream out "Nooo!!!" three times, once he realizes that Elysia is a trap and that he and the other Cthulhu Cycle Deities are going to be lobotomized by the Azathoth spawn that powers Elysia and is now going supernova and then transforming into a black hole.
Jim when the weights of his defense system don't fall properly, which almost rips Lilium's arm off. The system was very complex, as it was designed to both protect the gates of the village and uncover a huge hole under the wall. A malfunction wasn't only dangerous for the people setting it, but could also cause stress fractures in the material, and they were working against the clock.
The Fair Folk when La lowers her hood in the final battle. As fairies, their nature is to protect all children, and the fact that there was one in the Enemy's ranks, leading the army, nontheless, rendered them basically useless.