Some are puzzled by 'civil war.' How can a war be civil? A drummer of a band I was in stumbled over 'monopoly.' How can many be one? Exercise: find more examples of pseudo-oxymorons, and explain why they only appear to be oxymorons. Don't confuse a pseudo-oxymoron with such attempts at humor as 'postal service' and 'President Obama.'
Some consider 'jumbo shrimp' to be an oxymoron, but why? Can't there be big shrimp? I would classify 'jumbo shrimp' as a pseudo-oxymoron. Someone who considers this an oxymoron perhaps does not grasp that a big F can be a small G, just as a small H can be a big G. (A big shrimp is a small animal, while a small elephant is a big animal.)
Now if I were serious about this post, I would essay a definition of 'oxymoron.' But I think I'll take a nap instead.