Oh My Dead God!, it's used by the fans of Starship (a Starkid production) instead of the well know "OMG", it's sprang from the fact that in the musical "Starship" the science kill God, and it's a common joke in the cours of the play
when you're holding up your phone and making faces at it, as though you are taking a selfie, but you're really taking a picture of the person across from you or the wall or anything else that seems interesting but you don't want to be caught dead taking a picture of.
This action is often made more convincing by wiggling the eyebrows or opening the mouth, to pretend you're trying to get a Snapchat filter to work.