I assume you already know that a 75 gal tank is inadequate for a full grown adult oscar, unless you are willing to do almost daily, fin level water changes when it reaches that size.
I expect the 75 will become inadequate at about one year or so, because the oscars waste output will tend to overwhelm water quality rather quickly (using nitrate as an indicator) in such a small tank, and promote HITH disease as nitrate and other untestable nutrients build up.
This especially becomes relevant if your water is hard/mineral rich.