If it's just a single cyst, it's not. If it's lymphocystis, sometimes the cysts get a bit out of control, and will cover a large portion of the fish. I've never heard of it being fatal, but it will sometimes get in the mouth area and cause problems eating, which can lead to other problems.
Hopefully, you're just dealing with a single cyst.
Should the oscar start acting like it's bothering him, we might need to check into other possibilities, but as long as he's behaving normally, I wouldn't be overly concerned.