The original question... FOR THOSE WHO HAVE KEPT AN OSCAR...... obviously some respondents have not.
The original question was actually for those who have kept a single oscar in a 90g aquarium, either alone or with dithers...
Not that it actually matters, because regardless of whether the waste comes from a single cichlid in a 90g or a million tetras in a 500g the principles are still the same; you change your water based on the chemistry and test results. A single adult 12" oscar (because realistically that's about as big as they ever get) is a solid and messy fish, but it still isn't a huge load on 90g of water. I would put money on it that a single adult oscar (and a few dithers) would thrive in a 90g with 50% weekly water changes, provided your tap water is ok.
I have never done more than a 50% water change on a tank ever. Even grow-outs, or during treatment. I'll do 50% daily if necessary, but have never felt the need for more than that....