fish waste is ammonia (humans produce it too- its what makes pee smell). In a cycled tank good bacteria in your filter turn this into nitrite. Other bacteria turn this into nitrate. Ammonia and nitrite are toxic to fish, nitrate is less so in low levels. Some fish are more sensitive to ammonia and nitrite than others, osacars dont do well in ammonia and nitrite
When starting a new tank it takes a while for the bacteria to grow in your filter to turn ammonia into nitrite into nitrate. Thus is call cycling. So you need to get your filter running nicely with a good load of bacteria to turn the ammonia into nitrite then into nitrate and to do this you need a source of ammonia. You can either add something like stress zyme or you can put fish in that are not sensitive to ammonia and nitrite, like danios for example. You then test your water and once ammonia and nitrite are zero you can add oscars. However you will then be left with deciding what to do with the danios or they will eventually become food for you oscar
You reduce nitrate by doing water changes
An alternative way to cycle a tank is to get some filter media (the little rocks or hollow tubes) from another filter that is cycled and put it in your filter at the same time as adding your oscars, if you get enough media your tank will be almost instantly cycled