how do i take care of my oscar fish?

  Well oscars personality is like a pig its eats a lot and make dirt a lot.

So you need to water change weekly 50% and have good filtration.

Eating a healthy diet is good like bloodworms, hikari staple or bio gold, recommended is New life spectrum ..

Have fun with your oscar.

and 1 more thing they'll grow very big and fat so when they are adults 75gallon for single and 100g + for more oscars
keep with ur little pussy cat, they will become close friends
put it in water