Place it right after the surprising moment in an edit, then follow it with JoJo To Be Continued to close the scene like an episode ending. For a bigger build-up, open with Za Warudo Time Stop first, let the tension settle, and then let Joseph react.
About this sound
This is Joseph Joestar's drawn-out, theatrical cry of 'Oh my God,' lasting just under three seconds. The stretched vowels and sudden shock make it feel hilariously overdramatic, like a cartoon double-take at something unbelievable. It comes from JoJo's Bizarre Adventure, where Joseph's theatrical reactions became a running part of his character.