AN oxymoron is something that is a contradiction- two words that appear to be opposites.For example "conspicuous" is something you can see clearly and "absent" means missing.So how can he be both "obvious or clear" and "absent"? it means that the fact that the president was not there was very obvious.

Oxymora,  such as:   almost exactly,   almost certainly,  exact estimate, married bachelors  etc.,

are explained very well here:

http://www.drmardy.com/oxymoronica/oxymora.shtml