Kitten,
IMHO the 55 g is way to small for your oscar, even if he's by himself. Right now, while he's small, it may work, but in six or seven months he will be twice that size. A 55 g tank is 12 inches wide, this doesn't even give a full grown 13" oscar room to turn around.
Oscars are very powerful fish that will easily thrash an aquarium twice the size of your 55. I kept two in a 150 and they were always uprooting my plants, upending my slate mounted driftwood and just causing havoc. But you know, I really liked those two fish. They would eat food right from my fingers, get pissed when I was cleaning and attack my hand, or the vacuum, watch tv with me. No really, I made them some miniature recliners and they would kick back with a shrimp stick and watch Seinfeld.
They develop great personalities.
They do need good water maintenance. They are VERY messy fish and if kept in poor conditions are susceptible to hole in the head disease.
Recommended foods include beef heart, live fish, earthworms and night crawlers, which mine loved!
Oscars are a great fish to keep but I really would suggest keeping him in a larger tank. Again, JMHO.