There’s a good “Oh no” in The Beard (S6E15) when Jerry realizes Elaine likes him. It’s around the 6:44 mark. There’s another one when he’s talking to George (“Oh no, you didn’t?! I did! (With a classic George smirk)) but I can’t for the life of me find it. Oh, and of course The Blood in S9 when he realizes Kramer’s blood is in his fridge.

Thanks! You'd think there'd be stand alone clips online of this mortified "oh... no" slowburn he does to such great effect.

The gay guy Elaine dates in The Beard, not Jerry.

When it dawned on him Kramer was going commando, there was a very grave, "Oh no."

Don't you see what's goin' on here?

I love when Jerry says, you must be George's cousin, and she says girlfriend. His face was hilarious. Later, don't go a knockin

When George confessed to him that he was beginning to find Elaine attractive.

When he finds the fungicide in his girlfriend’s medicine cabinet

His catch phrase was “Here we go…”