Green’s Dictionary of Slang

agreement, go-ahead, approval.

c.1849		Paige Dow’s Sermons I 273: [Fortitude] infuses new life into his soul, while Hope adds an O.K. to his condition [DA].
1893		J. Hawthorne Confessions of Convict 241: He leaves and meets a pal who is provided with a check identical with the one presented, but having an ‘o.k.’ endorsement by the drawer written across it.
1917		S. Lewis Job 227: Still, he did condescend to ‘put his O. K.’ on pictures, on copy and proof for magazine advertisements, car cards [etc].
1928		‘J. Barbican’ Confessions of a Rum-Runner in Hamilton Men of the Und. 190: You had better not move the stuff until we give you the O.K.
1930		K. Brush Young Man of Manhattan 204: It needed his o.k., you see.
1938		D. Runyon ‘A Very Honorable Guy’ in Runyon on Broadway (1954) 421: I put the okay on you because I know you never fail to deliver a promise.
1946		Mezzrow & Wolfe Really the Blues 148: The colored porter [...] had gotten the okay from the bossman.
1953		J. Thompson Savage Night (1991) 116: I had to go to the dean of men for an okay.
1959		E. De Roo Go, Man, Go! 9: It had to have Beano Pierce’s OK.
1960		C. MacInnes Mr Love and Justice (1964) 202: Only your specialists can give me the final okay for your release.
1962		(con. 1953–7) L. Yablonsky Violent Gang (1967) 75: Blackie gave the OK and the boys joined me for a weight-lifting session.
1977		(con. 1960s) D. Goines Black Gangster (1991) 26: Nobody makes a move without the okay of their district leader.
1985		N. Pileggi Wiseguy (2001) 81: He had Paulie’s okay.
1987		W.T. Vollmann You Bright and Risen Angels (1988) 325: This was K.O. on the O.K.
2001		K. Sampson Outlaws (ms.) 112: That’s how come I had to ask for his okay in the first place.
2015		G. Disher Heat [ebook] His passenger would get out, secure the okay from the pickup point, then signal to the guard.