Green’s Dictionary of Slang

OK n.

also okay
[OK! excl.]

agreement, go-ahead, approval.

[US]Paige Dow’s Sermons I 273: [Fortitude] infuses new life into his soul, while Hope adds an O.K. to his condition [DA].
[US]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.
[US]‘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.
[US]K. Brush Young Man of Manhattan 204: It needed his o.k., you see.
[US]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.
[US]Mezzrow & Wolfe Really the Blues 148: The colored porter [...] had gotten the okay from the bossman.
[US]J. Thompson Savage Night (1991) 116: I had to go to the dean of men for an okay.
[US]E. De Roo Go, Man, Go! 9: It had to have Beano Pierce’s OK.
[UK]C. MacInnes Mr Love and Justice (1964) 202: Only your specialists can give me the final okay for your release.
[US](con. 1953–7) L. Yablonsky Violent Gang (1967) 75: Blackie gave the OK and the boys joined me for a weight-lifting session.
[US](con. 1960s) D. Goines Black Gangster (1991) 26: Nobody makes a move without the okay of their district leader.
[US]N. Pileggi Wiseguy (2001) 81: He had Paulie’s okay.
[US]W.T. Vollmann You Bright and Risen Angels (1988) 325: This was K.O. on the O.K.
[UK]K. Sampson Outlaws (ms.) 112: That’s how come I had to ask for his okay in the first place.