Get something smaller. The oscar will only last ~1 year and you'll have to do bi weekly water changes in a 55 once it gets around 7 inches because they poo like small dogs.
Yeah you need to just plan on not having anything like an oscar if your dad wont let you have bigger tanks. Just because you cant have a bigger tank doesnt mean you should have big fish. Find some other smaller fish you like.
This is my oscar from when i bought it a year ago till now. It was about 3" when i got it and roughly 11" now and is in my 125 gallon.They get big QUICK.
here are some p retty easy to findsouth/central american cichlids that IMO, would work in a regular 55gallon:
acara (Aequidens Pulcher) green terror female ( Aequidens rivulatus) convict (Amatitlania nigrofasciata)
Salvini (Amphilophus salvini)
Jack Dempsey (Rocio octofasciata)
Severum (Heros severus)
Ram cichlids (Microgeophagus Ramirezi)
Firemouth (Thorichthys meeki)
Discus (Symphysodon aequifasciatus)- not for beginners
Several of the "earth eaters"(too many to mention)
i would also like to recommend my personal 2nd favorites to cichlids, bichirs and ropefish. they are very personable fish. when i put my hand into the tank, my little senegal will swim through my fingers and let me pet her. i would suggest polypterus senegalus.
but i would like to stress as everyone before me FILTRRATION!!!! i have a cascade filter that is rated for a 100 gallon (265 gph)