It's an expression you use if you have a friend named; pronounced Stephen. It can also be spelled 'Steven'. Its a nickname for those guys that are goofballs, piss everbody off, but are still funny. Sometimes you even want to smack them, but they are still your friend.
He will make ridiculous jokes that make no sense, try to copy you and have a retarded laugh no one could copy and in return you say "Oh Steve".