First, I know that in some Platonic realm float oxymoronic forms that I have not yet made fixed and concrete. Let us hope that United Nations , once an oxymoron, may soon complete its journey to pleonasm. A few examples of these are Little Big Horn, Old New York, and Fork Union . And let us admit that the categories I have named are not always mutually exclusive. Old news, loose tights, and tight slacks tiptoe along the fuzzy boundary between natural and punning oxymora.
Civil engineer is both a punning and opinion oxymoron.
While the forms that oxymora assume are far from infinite, they are intriguingly varied. The boundaries separating one category from another blur and shift even as we draw them, but the lines can be useful. As all taxonomists should know, it is not always easy to know where the front of a horse ends and the back begins, but we usually can perceive the difference between a horse's head and a horse's ass.