Welcome to our little community.
I think you have already answered your own question when you say you need a bigger tank, this is by far the best way of keeping Oscar's healthy, providing them with a healthy environment to live in. if you can, I would advise you strongly to put on hold adding any more Oscars. Your tank is approximately 55 gallon, that is big enough for one Oscar only.
Seems like you have a good filtration. The key to keeping Oscars healthy are mainly keeping the water in good quality. Oscars really don't do well in poor water quality. Another important factor in keeping them healthy is their diet. A healthy varied diet is what we recommend, good quality pellets as their staple diet, with other little goodies as treats as we like to call them. Don't feed them on live fish. If you want to give them some live food then I would recommend things like worms, crickets, meal worms, all of these are much much healthier than buying fish that could potentially harbor diseases.
One very important thing to remember is that the Oscars are fairly high maintenance fish if you haven't got a large aquarium. That means regular water changes.
Anyway, I'm sure someone else will you be the benefits of their advice as well.
