Syn is short for synspilum and I personally wouldn't put anything in a 90 with a Oscar. I have 90 that is rapidly shrinking for me and my fish are only half grown. A 125 is a much better tank to house the two together
My pair of Syn's started out in my 90 with my Male ZZ Red Dragon. All was fine for the first 6 months but got progressively worse as the FH started beating up on the Male Syn but didn't bother the Female at all. The FH is now flying solo in a 55 gallon and the Syn's have the 90 all to themselves where things are calm again. I think your Oscar would work out fine with a Synspillum but you going to need bigger tank eventually if your going to keep them together.