Old Man Fart. When you're walking through WalMart and the old guy sitting on the bench leans over and rips a big one. That's an OMF.
Dude! Did you hear that OMF??
Do I smell like OMF?
Mom, that food smells like OMF. Gross.
OMF stemming from a typo while typing OMG. Similar in history to the creation of pwn from an own typo. Gennerally taken to have the meaning "oh my fuck".
OMF, I just shat my pants!
OMF, Tommy really likes to take it in the butt.