God my Oh! is simply the reverse of Oh my God!.
and still means the same.
most of the
time when it is used the commenter is trying to be
random.
The origin of this word/catchphrase came from an episode of an
anime called: Panty & Stocking with Garterbelt.
Just before a character got blown up he said: "Oh my God!"
but when he was resurrected (the scene of him blowing up in reverse) he said "God my Oh!" instead of "ho ym dog".