It seems many people don't believe a fish needs any exercise to be healthy.
And in truth a 120 gal long tank, seems only 30 gallons more (not much) than a 90.
But if a fish can only swim two or three of its body lengths the long way, and barely more a body length the breadth of the tank, not much in the way of exercise.
Whereas a 120 or similar length tank provides almost double that swimming space the length of the tank, quite an improvement (at least in length).
We wouldn't keep a Great Dane or Akita cooped up in a small bathroom 24/7,
the similar equivalent of a 15" fish in a 4ft tank.
Filtration has nothing to do with nitrate buildup, only water changes can dilute that, so to me a 50% water change once or twice per week is rather a rather wimpy regime for a heavy waste producer like an oscar.
I think once it becomes an adult ,50% water changes every other day would be needed in that small a tank, no matter how adequately the filter works, or the common malady HLLE, that oscars get when too confined, will be the result.