One oscar should be in more than a 55 long term. For now it is fine if you do enough water changes, but once it hits 8-10" you really need to upgrade.
IMO oscars should be in no less than a 75, and that is minimal for one to two. They can easily hit 12-15" if properly cared for (which is not the case when in a 55).
Your water changes need to be enough to keep the nitrate concentration under 20ppm BEFORE a water change.
Diet is vital with all fish but extremely so with oscars becuase they are so prone to Head and Lateral Line Erosion (HLLE). I recommend New Life Spectrum, btu whatever you feed it has to be the highest quality food possible. Oscars in a 55 is the classic situation to cause HLLE. HLLE is also caused by water quality, again so vital with oscars.
With the 55 I wouldn't ad any more fish. If you really take great care of the tank and upgrade at about 8" you could do large bristlenose plecos for algae control. Other than that it really depends on what tank you upgrade to. I would never suggest common plecos, they get way too big, produce massive amounts of waste, and are not very effective at algae control. If the tank is large enough other relatively passive cichlids could work. Other common tankmates are Synodontis spp., Bichirs (Polypterus spp.), medium sized catfish, silver dollars, and many others. However, any of these would require a big upgrade (at least a 125). If you do not ever upgrade larger than a 75 I would not recommend anything more than two oscars.