The narrator describes Colonel Cathcart, probably the most reprehensible of the many distasteful military leaders in the novel, rarely in the novel. This, from Chapter 19, is one of the fullest descriptions of him. Like many of Heller's descriptions, it is self-contradictory:
Colonel Cathcart was conceited because he was a full colonel with a combat command at the age of only thirty-six; and Colonel Cathcart was dejected because although he was already thirty-six he was still only a full colonel.
Heller uses many different types of self-contradiction in the novel. This instance is an oxymoron, since Cathcart is described in two opposing ways about the same thing. He is both dejected and conceited over the fact that he is a colonel at age 36. He is feeling a dejected conceitedness, a conceited dejectedness; feeling both these things at the same time is an oxymoron.
Note that this is different from the paradoxes in the rest of the book: there is no logical contradiction here, just an emotional one. And note that this isn't an instance of irony, which Heller uses often in his self-contradictory statements, either: the narrator here says exactly what is meant. Heller uses many different types of contradiction. All of them expand his absurdist depiction of the war. In Heller's version of wartime life, nothing makes sense, and everything contradicts itself in many different ways.