Green’s Dictionary of Slang

1. (orig. milit.) one’s opposite number, best friend; in weaker sense, a companion.

1922		‘J.H. Ross’ Mint (1955) 163: He shifted Clarel up the hut to be my oppo.
1945		D. Bolster Roll On My Twelve 137: Oppo ... short for Opposite Number, the man in the other watch or in another ship who carries out the same duties as yourself.
1951		T. Driberg Best of Both Worlds (diary) 24 Apr. (1953) 18: O’Brien from Harrow [...] whose ‘oppo’ Ralph Haine [...] is, at twenty, eighteen months older.
1965		(con. 1940s) B. Behan Confessions 127: I find myself sexually attracted to my oppo.
1967		J. Wynnum I’m a Jack, All Right 8: You’ll have to smack it about if you want to be my going-ashore oppo.
1975		‘P.B. Yuill’ Hazell and the Three-card Trick (1977) 52: Tel wanted me to go with him and his oppo Rodney for a light ale.
1983		A. Payne ‘Get Daley!’ Minder [TV script] 62: He’s Daley’s oppo.
1984		(con. WW2) T. Jones Heart of Oak [ebook] A particular friend, if he was about the same age, was an ‘oppo’. If he was younger, a ‘winger’.
2004		N. ‘Razor’ Smith A Few Kind Words and a Loaded Gun 38: You and your oppo there are at it, ain’t you?

2. opposition.

1989		J. Morton Lowspeak 108: Oppo – the opposition.
1995		J. Cameron Vinnie Got Blown Away 76: Thin out the oppo early on.
2001		K. Sampson Outlaws (ms.) 166: Who you sending down as the oppo for Mikey’s men?
2008		E. Drew in N.Y. Rev. of Books 17 Apr. 14/4: Every Campaign has ‘oppo’ (for opposition) research.
2013		K. Sampson Killing Pool 173: Well, if it isn’t my delightful old oppo McCartnety [...] the worst copper in Liverpool.

3. opportunity.

1999		J. Cameron Brown Bread in Wengen [ebook] Never such an oppo for mashing up some boxers.
2002		J. Cameron Hell on Hoe Street 19: I never took the oppo for stroking up her tits.