Its not just about swimming space.
There is also how fast the water becomes oscar urine soup.
A full grown oscar puts out tons of waste, in feces and urine.
And although mega filtration will help a little, the only thing that dilutes the urine, is water changes,
If you are willing to do every other day 50% water changes, a 90 gal might work. And as an oscar grows those changes, must also grow, and become more frequent.
If you are one who can only change 25% once per week, by the end of that week, the tank might be living 50% oscar urine, which leads to chronic disease, and the typical semi adult oscars we see turned into LFSs with hole in the head scarring, gill curl, and other poor water quality maladies.