its alright but my research led me to belive oscars prefer it a tad cooler 79 f being a little better they come from deeper water they also prefer the tank on the darker side but like any fish they will aclimatize to your tank if 81 is easier for you then it wont be a problem.
I've also heard that increased temp leading to increased metabolism can lessen the fish's lifespan. probably not by much because 81 isn't that high but personally I'd lower it a little to around 78.