That one guy who is in that one girl's life who foils your plan of getting with her. Other Guy is not a person, but more of a position. Other Guy can be a close guy friend, an ex-boyfriend, someone trying to hit on her, etc.
"Dude, did you hit on Sarah last night?"
"I was going to, but then Other Guy came and stole my thunder. She was talking to him the whole time."
"Aww, man. I hate Other Guy!"