Welcome! lol... and The Oscar will grow Quick in the 55. There are alot of SA cichlids that would work well w/ an Oscar. Personally I'de grow-out the Oscar in the 55 then when you've got one of the larger tanks set-up put him in w/ some other SA's. The Oscar alone in the 55 will out-grow it in less then a year. But they make a very interactive "larger" fish. Oscars also do well with larger less aggresive tank mates that will still hold their own. Most Oscars act tough, But push come to shove even if the largest fish will get bullied fairly easily as a general rule.
And Contrary to popular belief Oscars are imo not an easy fish to keep. they do heck to decor, and filter systems. They are very dirty, very mischevious, But worth the headache imo. Given the encouragement a very intelligent and personable fish.