Oldest oxymoron I know is glykypikron = "bitter-sweet", as said of love by the poetress Sappho about the beginning of the 6th century B.C.

A fine translation by J. Addington Symonds, 1883:
Lo, Love once more, the limb-dissolving King,
The bitter-sweet impracticable thing,
Wild-beast-like rends me with fierce quivering.

In friendly fire I can see no oxymoron; friendly here only designates an origin and no intention. In the tis man's Army it is well known that artillerists don't distinguish between friend or foe; the bloody bangheads only know interesting targets. A long range hit at a friendly officers latrine they consider as a ballistic masterpiece, too.

Not so funny is the intelligent drummer. I only agree if you except me personally.

And now to Greg: There are certain hints at the use of ironic humour in the U.S. of A. Even the British Library shows six entries of this excellent and entertaining work:
Title: Confederacy of Dunces
Author: Toole. John Kennedy
Publisher Name: Louisiana State Univ Pr.
Date of Publication: 1980
ISBN: 0807106577
Shelfmark: 81/6110.
Status: missing
Note the status: It must be considered so good that it was not brought back by a customer.

Wilfried