Green’s Dictionary of Slang

(US) all right, in order.

1916		Seattle Star (WA) 11 Nov. 8/3: Key to Broadway Slang! ‘What do you say to papering the hoop and kicking the gong?’ ‘K.O.!’.
1922		Broadway Brevities Aug. 20/2: But the best story of all — now told k.o. for the first time — is how Dave gave the air to his old girl, Betty Hudson.
1927		H.C. Witwer Classics in Sl. 84: ‘That’s K. O. with me,’ says the goofy Antonio.
1929		H.C. Witwer Yes Man’s Land 26: The girl’s kayo and will get places. [Ibid.] 163: ‘Kayo,’ agrees Jack.
1962		Ragen & Finston World’s Toughest Prison 806: kayoe – O.K., ‘all right’; a knockout.