Oxymoron is a strange little mouse living in the Bookland at Exclamation Point. Everything he says is an oxymoron, such as "As an exception, you always should [have a clause]"
As a result of his nonsensical nature, Oxymoron is something of a jester and heckler of the Bookworm's.
As the Bookworm says; "Never mind Oxymoron and Diphthong. They're limited grammatical principles, you know."