Are girls that say,"Oh My God" all the time more likely to have a tramp stamp?

Never entertained the thought! I work in a tattoo studio, and put on quite a few lower back tattoos. Never heard a connection with that particular figure of speech! (by the way, we don't call 'em 'tramp stamps'. That's a derogatory term dreamed up by some narrow-minded geek who wasn't getting any) But nothing wrong with OMG!