My Oscar is about 12" long at two years in December.
What kind of fish should I be looking at getting him for his Birthday... Companion, not dinner!!??
Any suggestions?

How big is your tank?

Sorry...LOL
It is a 55 gallon.

introducing a companion for a fully grown oscar is a hit and miss thing.
he used to be alone and any new comer will treated as intruder.....
tank size and filtration capacity is needed to determine if a tankmate
can be accommodated.

I have 900gph going through my tank now (55 gal.) as suggested to me by the petstore because they are so messy and he eats the cleaner fish I buy.

you could put in something small like a Synodontis catfish, but you'll be pushing the limit on your bioload, as the O alone can use the 55 Gl for sure.
But you got some major flow on you filter, so that helps a bit.

You dont have much room for any other fish. I would leave the tank as it is and not add any more fish but if you must add another fish add something that doesnt grow bigger than 6 inches and also make sure it is big enough not to get eaten