Green’s Dictionary of Slang

(orig. US) one’s steady girlfriend.

1928		H. Brackbill ‘Midshipman Jargon’ in AS III:6 453: O.A.O.—The ‘one and only’ girl.
1932		‘Patois of Annapolis’ in Sheboygan (WI) Press 17 Sept. 8/3: The social functions arranged for midshipmen in ‘Crabtown’ are ‘hops,’ and when a middy takes his ‘O-A-O’—the one and only girl—to the ‘hop,’ he is ‘dragging a girl.’.
1947		J.L. Riordan ‘Some ‘G.I. Alphabet Soup’’ in AS XXII:2 Apr. 111: A soldier’s ‘one and only’ girl is his O.A.O.
1968		Current Sl. II:3 5: OAO, n. The one and only.