It is not an oxymoron, it is a paradox, duly noted by Simon Leys an others: Only the ones with the strongest personality (hear "self") can truly wipe out their ego. For us normal people, the goal could be to lower expectations about oneself, but for those rare birds, usually monks, I believe it means something very deep (not much related to PB's note, though).
You're using "ego" with that "vanity" sense. I'm referring to ego as in "self". I agree that strong people have less "ego", less "self-importance". Try wiping out "your self" instead and see those strong people crumble.
