I am currently only using an undergravel filter for my oscar. There's only one Oscar and its about 3 inches long. Is this good enough or should i get another filter?

I definitely think you should be looking for extra filtration before your Oscar gets too much bigger. Under gravel filters are okay for community tanks, but when it comes to large messy fish that produce lots of waste, you run the risk of them becoming clogged up. I would strongly advise you to get yourself an external canister filter that is rated higher than your tank. This way you can pack lots of biological filtration into it plus enough mechanical filtration to catch all the solid waste.

I agree with ofl, an external cannister filter (that is rated for at least 1.5 times the volume of your tank) is needed for keeping oscar.

The problem with undergavel filters(UGF) is that they can clog up with oscars mess. Also oscar like to dig and move gravel. If the oscar moves gravel and exposes the plate then all the water flows through that gap and not the gravel and then the UGF dont work

Ok thank you for your advice i shall get another filter