Place it right at the peak of an unexpected reveal in an edit, then follow it with No God Please No for a bigger reaction, or cut to Okay Let's Go when the scene moves on to what happens next.
Über diesen Sound
This short clip repeats the exclamation omg over and over in a high, breathless voice, stretching a single moment of surprise across about four seconds. It feels frantic and funny, like someone who cannot quite believe what they have just seen, and it works best as an instant burst of disbelief rather than a calm reaction.