Hello; One of us appears to have missed some of the important information in the is thread. My take is that the tanks is very newly set up and is not yet cycled. If my take is correct there are no nitrates as yet. Nitrates come some weeks later after the beneficial bacteria (bb) colonies become established.Oscars are extremely robust. They'll probably laugh at the nitrites and grow a few inches DURING the cycle.
The fish make ammonia as a metabolism byproduct. That ammonia stays in the water until enough of the ammonia reducing bacteria grow an established population and can then change it into nitrites. Then eventually a population of another type of bacteria that can convert nitrites will grow. This second type can then turn the nitrites into nitrates.
So if I am mistaken and there are nitrates in the tank then the tank must be "cycled". And yes nitrates are much less toxic and can be tolerated by fish while ammonia and nitrites are damaging at even low levels.