Its "funny" how this thread always seems to develop. It just keeps coming back over and over.
You will get one of two answers on this yes or no. And alot of reasons why. I personally believe that a 90 is to small in width for a full grown Oscar to be happy for life, but like I said and others have also with proper water changes and filtration you could do it. You can do alot of stuff but I wouldnt recommend some of it. You could live and survive in a 3×3×8 room but you wouldnt be happy. If we doubled the size youd be more happy. Until we became roommate's but may be happier then in the 3×3 still.
Can you keep sn Oscar a GT and some Acara? yes. Will you hate life with all the waterchanging? probably, will you have casualities? probably, will it be easier for your fish to get sick? possibly. I have also seen the oscar in a 55 and in a 75 and in a 90 they seemed fine......
I have also seen a solo oscar in a 120 and a 180 those were the best looking Os ive seen ever. A true wet pet happy to see people no hth no scars no tattered fins.
So the point is can it be done? Yes. Should it.......?