I agree with adding a filter ASAP, till you do you must do large scale daily water changes. When you do get the filter put media in it from another filter, this will have the bacteria you need to get rid of ammonia and nitrite, also adding aged gravel fro manother tank will help. Also keep a close eye on your ammonia, nitrite and nitrates, check them daily. If the tank is only about a foot long then I suggest getting something to replace that as well. You can buy 30-50g rubbermade bins at Wal-mart for something like $7.00. They make great cheap growout/QT/hospital tanks and will take a lot of pressure off of you and stress off of your oscar.
You DO NOT want to try and keep an oscar is a small uncycled/unfiltered tank for very long.
