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1.
Origin: My damn mouth

Oh my damn is a constantly used term to replace saying such as "Holy Shit", "WTF", Ect. Though it has a different while still similar meaing, it expresses extreme surprise
oh my damn theres a gartian in the back seat.

oh my damn she took the whole thing
by Tra la la December 27, 2004
 
2.
i word usualy used when shocked or surpriesed
to express excitement
brenda:i just got with travis
me:ohmydamn, brenda's a PIMP :D
by lifeonthemurderscene January 01, 2009
 
3.
When a guy sees a very HOT girl. The phrase is normally said to other guy friends as a secret guy code.
A.K.A. "OMD"
Franky "Oh crap do you see that girl over there?!"
Jonathan "Oh my Damn!"
Franky "Agreed"
by volcomishockey November 26, 2010