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Personally if you are happy to spend the extra time cleaning I'd go for sand, oscars profuse a lot of waste so if a quick and easy gravel vac is what your after then gravel is your best option
If you are set on a substrate I'd advise either river stones about the length of a fist or fine sand. I agree with Brookster123 on this, bare bottom is probably best.
However, whatever you do I would avoid getting gravel for your Oscar tank. I had a friend that kept an Oscar in a gravel substrate tank. The Oscar accidentally killed itself when it gobbled up a pebble alongside a pellet of feed. They were broken up by it.