I would most definitely not put in another oscar. Some people will tell you, yeah, you got room, but 2 full size oscars in a tank your size just isn't nice. It sounds like you're pretty set already. I think your fish's health will be better if you don't overcrowd. I have a 125 and alot of people think I could put more fish in, but everything is going gravy, so I don't rock the boat. I did have an oscar in with them, but he got too big, so we found him a better home.
Its's all about what you're comfortable with. If you think you still want to add, severums would be a good choice with what you've already got, but they get big too. The list for my 120 below had an 10in. oscar in it, minus two kissers.
A side note--Any fish you add need to be a good size already, as it will be competing for space. Parrotfish can be testy too, can't they?
