the tank is too small and has a huge amount of bioload, so until you upgrade it is going to need about 75% pwc
per week. they also need high quality food, good temperatures and the filter pads need rinsing in a bowl of old tank water so they are clean but have all their BB.
for 2 oscars a 75 is a minimum for they hit 12in, and often keeep growing. and the fact you have 2 oscars means a likely chance of a male or two, so a fight to the death may ensue when they mature.
im sorry you will get a lot of flak for the tank, but its sorta not fair for the oscars xxxxx