My oscar recently for a couple days has been inactive barely swimming and usually laying in a corner. For the most part my water parameters haven’t changed and he’s been in worse. I do feed live fish so maybe? Not sure what to do
Some more info will be needed.
How long has the tank been set up? Was it truly cycled before you added the oscar?
What size tank? How often, do you do water changes, and how much water is changed (%)?
What size oscar?
Do you quarantine the feeder fish before using? Where do they come from?
What type feeder fish?