As always, It's really hard to tell from the picture. That to me certainly looks like a bacterial infection. But you're right there, so take a good look at the white patch near the mouth. Is it raised off of the skin? Does it look fuzzy like cotton candy when you look at it from an angle? If so then it's a fungal infection. Luckily Oscar's a pretty tough fish. In most cases this wouldn't be a critical situation. You need to determine whether this is bacterial or fungal. Then treat appropriately. I'd stick with the lighter meds at first. Melafix for bacterial and Pimafix for fungal.
Also, and this is not a side note, check your Nitrogen levels! Ammonia, Nitrite, Nitrate. Oscars usually don't get sick if their water is in good shape and they are keep on a healthy diet.
Bottom line, check you water, determine the cause of the infection, fix the water if necessary, then treat the infection.
