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!"
The guy who answers the phone of intended recipient, only to explain that he is not the guy you are looking for, but the other guy. Heard often with non-native speakers who command broken English, particularly of the Indian sub-continent.
Fash: Yo Bils, wassup my main geezer!
Other guy: This is not Bils. This is other guy
Fash: No shit? Where's Bils?
Other guy: Bils not here. This is other guy.
Indian, Pakistani, Bengali, Freshie, Broken English