"OMD" is an acronym for "Oh My Dog". Generally used as an expression of disbelief or surprise. It originates from You Tube video "Oh my dog... - talking cat" (http://www.youtube.com/watch?v=9LBKVXyrHcw)
Variations include "OMF" Oh My Fuck, which describes a state of anger or frustration. And "OMFD" Oh My Fucken Dog, which is a mixture and hightened expression of shock, anger and disbelief.
OMD, i can't believe you just did that.