Green’s Dictionary of Slang

(orig. US) an orchestra, usu. a jazz or dance band.

1935		Indiana Eve. Gazette 20 Mar. 9/2: Variety [...] was the first to call an orchestra an ‘ork.’.
1935		M. Fulcher ‘Believe Me’ in Afro-American (Baltimore, MD) 20 Apr. 5/1: Fred Norman [...] useta toot in the school orch.
1936		A.E. Duckett ‘Truckin ’round Brooklyn’ in N.Y. Age 12 Dec. 7/1: The well-known ivory tickler [...] is holding forth with a grand orky.
1938		P. Grey ‘’Twixt Night ’n’ Dawn’ in Afro-American (Baltimore, MD) 1 Oct. 10/6: Jimmie Jenkins and his orkie.
1940		Pittsburgh Courier (PA) 6 Jan. 9/1: Fletcher Henderson gives up his ork to go with Benny Goodman.
1959		S. Bechet Treat It Gentle 88: There I was, working in the Eagle Orch [HDAS].
1982		A. Shaw Dict. Pop/Rock.