No need for an expensive led light and a fx5 or fx6 will work perfect because you will have to get a bigger tank within a year or so anyways because the Oscar will grow to 12"+ in about 12 months so you can use that filter on your next tank too. I would definitely use 900gph+ of filtration for an Oscar in a 55 or 75g tank
Yes a FX5 or 6 would be great on your 55g oscar tank. As stated above, you'd be off putting that money towards a tank that will support a full grown ocsar.
What do you have for filtration and lighting currently on your 55g tank? How big is your O?