Fighting those two damned claw-things in RE4 for the first time. Without any knowledge of the bells whatsoever.

Rounding the corner in Team Fortress 2 and seeing a heavy on the other side with his minigun spinning.

Gears of War: Warzone Match, seeing a teammate's head splatter like a melon next to me. I then try to flee the scene but ran into a chainsaw. Oh shit x2.

The Elder Scrolls IV: Oblivion: At level 5 I was wandering around and decided to explore any random cave I would encounter. I found the one chick wearing Ebony Armor inside one of the caves. Thought I would have an easy kill after sneaking up on her. I died.

Guilty Gear XX #Reload: Fighting a round with Gold Sol using any normal character(No black or gold) at the highest difficulty.

DOA5: First time fighting Alpha-152. Any rounds following the first quickly became less "Oh shit" and more "FUCK YOU"

The nuclear explosion in COD4 and its aftermath.

Watching a CD disappear in SSBB. They only stay for about five seconds.

Also, any of the numerous times dogs come at you from nowhere in CoD4. Hate them so much.

Super Ghouls 'N' Ghosts: Every god damn second

Pretty much anytime a grenade or flashbang gets close to you in Counter Strike or CSS.

A bunch of times in Condemned.

When I climbed over a chain in the subway station, only to be immediately smacked upside the head, sending me down an escalator to fight 3 hobos.

When I first saw the crawly dudes in the subway tunnels...creepy.

When I saw three squatters in the library gun their way to the elevator...that I would have to take to the next floor.

When I smacked one of the manikins...and he screamed and died.

When the hobo bursts through the drywall and grabs you.

Where do I begin... (Spoiler Tags to both avoid spoilers and save time for those who don't want to scroll past my long-windedness)

In RE4 when your walkin through the kitchen on the island, and outa friggin no where a guy explodes out of an oven next to you. Scared the ba-jesus outa me...

'would you kindly'
when it was revealed i was like 'WHAT THE @#$%'

When I accidently shot Big Daddy.

when im walking to the end of the room that has the (insert important item hear) in it and the a giant fly thing or something comes outthen you half to beat it and on your 80th try you finally beat it just to find out that its not all the way dead and it takes the iem and frys off or you find out that it wasnt in that your.

whenever I play games where boss battles include QTE's. Tomb Raider Anniversary... TREX.. *twitch*

I think the biggest Oh Shit moment was when your playing Starcraft and youv used all your resourced dog fighting your friend with marines and vultures, only to see interceptors on the horizon

on half life 1 wen i saw the last boss ( the big genital thing)

racoon39:
when im walking to the end of the room that has the (insert important item hear) in it and the a giant fly thing or something comes outthen you half to beat it and on your 80th try you finally beat it just to find out that its not all the way dead and it takes the iem and frys off or you find out that it wasnt in that your.

Someone speak this language? I have no clue what this fool just said.

I've got an old-school one: Quake. One of those flesh-colored things with hooves and horns is on the other side of a crevasse from me. I'm just like "Oh, he can't hurt me from over there", and then it LEAPS across the gap and proceeds to tear me apart.

Playing Urban Dead, running out of AP, knowing that zombie ass-rape is imminent.

My first time through RE4, I missed the file that tells you how to kill the regenerators. After wasting god-knows-how-much shotgun ammo on the first couple you meet, I gave up on being able to kill them at all. Then later, in the freezer (you know, the one with seven or eight frozen regenerators lined up in it), I came to the grim realization that the only way to get the apparently-important item at the back was to SHUT THE EFFING FREEZER OFF!

What actually happens is nowhere near as bad as what I was expecting, but my initial reaction was definitly "Oh, Shit."

portal: "i am going to murder you. ha ha." as your heading for the incinerator
Halo 3: Cortana + legendary
Tales of Symphonia: when Regal broke his chains and blocked the passage as the angles (bad guys) descended on him

"Hello, Clairice."

Just remembered another, even older one- the original Doom (shareware version at that) gave me not one, but two; the first was the first time one of those big pink shaved gorilla demons showed up, I think on E1M3; the second was when I encountered the Barons of Hell on E1M8 for the first time. Both times I nearly jumped out of my skin... Which is pretty much what the game designers intended.

Kingdom Hearts: Sephiroth. Hmm...Platinum Match, eh?...Oh damn.

I would have to say my biggest "Oh Shit!" moment was in Doom 3 when the gray, fast bastard with no eyes jumps from under the yellow stairs you're about to go up and into your face. First I soiled myself and then I had to play Simon the Sorcerer for a week because my heart started crying everytime I saw the Doom 3 logo on my desktop.

Neflame:
Anytime in the Marathon Trilogy when I encountered a Juggernaught. Those things don't screw around.

Oooh, good one! Complete agreement! Also, reading up on all the complexities of the backstory - that really impressed me, no one else has ever done a job like that of building up a full functioning world - and then only communicating it through text on the screen (the terminals). Amazing.

Another Marathon-related one: Playing Marathon Rubicon and realising just how f*cking good it is. That really was a cool gaming moment.

Deus Ex (spoilers_: Breaking out of the MJ12 base, and as I head along the corridor, I'm thinking "This bit looks familiar...." and then I run into the hidden area of the UNATCO base, through that door I hadn't been able to open before.

A couple of days ago in EVE Online when I got careless and jumped into low-sec with my defenceless industrial ship--and upon arriving, saw about ten battleships, assault cruisers and other nastiness, all pirates. As the post-jump invisibility runs out and I'm starting to panic, they all choose that very moment to get bored of waiting and head off somewhere else. How's that for luck?

In Thief 2, running across one of those strange boy-like robots, looking down at the body of someone I'd knocked out earlier. I crept up and gave him a smack with the sap. He turned and made this....inhuman noise and I just panicked.

Mr.Expendable:
I would have to say my biggest "Oh Shit!" moment was in Doom 3 when the gray, fast bastard with no eyes jumps from under the yellow stairs you're about to go up and into your face. First I soiled myself and then I had to play Simon the Sorcerer for a week because my heart started crying everytime I saw the Doom 3 logo on my desktop.

That reminds me; at the end of Doom 3 when you walk through the hole in the wall, take a few steps forward and hear a roar behind you. You turn and realise that you just walked out between the legs of a several story tall Cyberdemon! I was about to change my pants when I took it down in the first go and realised it was a bit of joke actually.

And for all you arachnophobics like me, the ice base in RE:CV when you can see that massive spider crawling around on the underside of the ice an you just KNOW that you are going to have to fight that big bastard sooner or later. Makes my flesh crawl just thinking about it.

First time i played Halo 1 and the flood came out, i ended up running and chucking a plasma grenade onto my mate, yeah he was dead and i ran round with a load of those creatures on my head like i was having a seizure.
Same with the old alien trilogy game when those things would just pop up on the screen just sucking your health away.
Also FF7 when your facing sephiroth at the end and he's doing those powerful moves like supernova which wipes out your partys health

I definitely have to say when I watched Yui give Citan his sword in Xenogears. It wasn't a bad "oh...shit." It was one of those ones where you realize one of your most OP characters...just a little more OP.

Even though it's been said many times: the last test-chamber level in Portal. But not just because of what happened when I rounded that corner (which took me several tries, by the way, I just couldn't get the angle right for some reason). What burned respect for that game into me was the pitch-black flash of realization that this is what that semi-cute, fully-creepy AI was telling me would happen from the get-go.

Also: the hallway in F.E.A.R where you're walking along, minding your own business, and suddenly you realize that the even-more-creepy-yet-not-at-all-cute little girl has been standing in your path, staring at your sorry arse, and you hadn't even noticed.

Having to walk through the hallway full of guards that Cyborg Ninja just killed in Metal Gear Solid.

AvalonSkies:

Also: the hallway in F.E.A.R where you're walking along, minding your own business, and suddenly you realize that the even-more-creepy-yet-not-at-all-cute little girl has been standing in your path, staring at your sorry arse, and you hadn't even noticed.

good one, thats a pretty scary one yaeh, most of the time she doesn't do anything until a few moments later, so at that point you will almost immediately know wether you're screwed or not.

BlackOps:

AvalonSkies:

Also: the hallway in F.E.A.R where you're walking along, minding your own business, and suddenly you realize that the even-more-creepy-yet-not-at-all-cute little girl has been standing in your path, staring at your sorry arse, and you hadn't even noticed.

good one, thats a pretty scary one yaeh, most of the time she doesn't do anything until a few moments later, so at that point you will almost immediately know wether you're screwed or not.

And when you are climbing down the yellow ladder, you walk up to them and press the action button and you turn arround to go backwards down the vertical ladder and the f**king bitch is standing right behind you and all you see are her feet!..

CoD4 getting close on the hillicopter to fire. then turning around becuse well your too close. (my dad was beside me yelling hahaha run! run!your gonna die!)

actually that level was about oh.. shit... all the way through.

Two bits come to mind:

The first was in a survival horror game called Obscure, when some plantmonster thing knocks a wall down practically on top of you.

the second?? Get ambushed on vegas 1, pull my pistol, run out and start shooting.. well, i woulda liked to start shooting, all i got was a "click,click"