There's nothing wrong with the occasional patting of an oscar if he allows it.
You tube is flooded with vids of folks doing that.
But keep in mind that any touching of the fish may damage it's protective slimecoat.
But as long as it's not a regular and continuous thing, imo you can pat an oscar.
I did pat mine once, but he came back after a minute and bit me hard, as if to say, don't you get any funny idea's!
Another thing to consider is, that the lateral line in oscars is a very sensitive organ, to enable to make the oscar feel vibrations in the water and i am told that touching the lateral line would be like touching our eardrum.
So you better not do that...
My oscars don't like to be touched, but i got my Gar who seems to enjoy an occasional pat on the back.
edit: as i say You tube is flooded with it, do not see that as an encouragment to do anything that people on youtube do...