Carly, please stop asking for advice if you are not prepared to listen! You seem to have made your decision but, if it were me, I would be making sure I had explored every avenue before I made an irreversible decision
I realise this is a stressful situation but everyone on here has offered you good advice which you have not taken - we are just going round in circles here and you and Oscar are just becoming more stressed in the process
If you are willing to try everything, you need to get an opinion and further tests (as recommended by Lurchergirl) from a vet AND an experienced behaviourist eg Mark - and you need to do it now!
You keep asking what a behaviourist will do, say, recommend - the answer is,
we do not know until they see Oscar.
As has been said, it is unlikely that a rescue will take him with the facts as you have told us, but either you want to try everything or you do not. Obviously an inexperienced home is totally out of the question for him so, if you do not want to talk to a behaviourist, it looks like pts is the only outcome for this poor boy