Oscars real father....
Could actually be Louis Tully, in part one it was heavily implied that the Keymaster (Louis) and the Gatekeeper (Dana) had to have sex in order to unlock the gate for Gozer to come through (plus the sexual innuendo keymaster/gatekeeper). Other ways it was implied was when Peter was at Dana's apartment in part one and she thinks he's the Keymaster so she is trying to seduce him (e.g. asking if he wants that body, etc). Plus in one scene right before Gozer is introduced, you see Dana and Louis waking up right next to each other, and he's got this grin on his face.
Doing a little of googling, there was apparently a deleted scene where Louis is actually asking Dana if they did it while they were both possessed.
So we now know who Oscars REAL father is!