An acronym for oblivious to the obvious. Usually used as a verb, but can be used as a noun.
(noun) In retrospect, Nate realized he was being an otto after finding out she was a he. He was relentlessly jeered by all who knew of the incident for the rest of his natural born life.
(verb) Is it possible to be so otto as to not notice the adams apple, or the bulge in heshe's dress for that matter?