I would look into a 125, I have one. I love it, it's not crazy big like some people have but big enough for some "big" fish. The main difference is it's 6 feet long instead of 4 feet. Would fit along a nice bare wall, a piece of furniture really.
In addition to what packer43064 has stated, having a 6 ft. tank gives the Oscar more room to move around in, which is what they need. I had an Albino and a Tiger Oscar in separate 55 gal. tanks along with a couple of Banjo Pleco's (one in each tank). I put of one each in both tanks when they were small around 4 to 5 inches each. By the time they were 8 to 10 inches in size (4 months later), they were all in need of a larger tank. Granted, I could let them grow and continue to stay in the 55's, but I didn't want to stunt their growth. So I started looking for larger tanks. Living in a backwoods town at the time, it was impossible to get anything larger than a 55 gal. shipped in from anywhere through my LFS.
How long have you had the 75 for? I would just skip on the Oscar if you wanted more fish to a tank. Obviously buying a new tank is great but if you just want more fish the 75 is good for alot of fish.
This is great advise from packer 43064. If you really must have the 75 gallon stocked, I would choose cichlids that range from 3 to 5 inches in size as adults - or - as being fully grown. If you do this you can have a small variety of hopefully compatible cichlids, and your tank will not be overstocked. Keep in mind to, your bottom dwellers will have to be included into your tank stock inventory.
To begin with maybe, but in reality that Oscar in a few years time will be 14-16 inches topping out. Believe me, it's small now but it'll grow quick and the tank will look small with even one Oscar for a 75.