Green’s Dictionary of Slang

1. (US) unmarried.

1931		D. Runyon ‘Romance in the Roaring Forties’ in Runyon on Broadway (1954) 39: She thinks I am single-o.
1956		(con. 1940s) J. Resko Reprieve 234: He was a zoot-suiter and [...] single-o fancy man.
1957		N. Algren ‘G-String Gomorrah’ in Entrapment (2009) 201: The single-O solitary with paws wrapped tight around his dollar bottle of Budweiser.

2. (US Und.) of a criminal or criminal activity, working alone, solo; also in non-criminal contexts; also adv.

1930		R. Chadwick in Liberty 5 July 20/2: I have my first experience in single-o jobs.
1953		T. Runyon In For Life 197: When I went it would be strictly a single-o deal.
1955		D. Maurer in Cressey & Ward Delinquency, Crime, and Social Process (1969) 831: A lone pickpocket [...] is usually referred to as a single o tool, a single handed tool, or a single o cannon.
1971		T. Thackrey Thief 291: I had pulled it off single-O.
1972		B. Rodgers Queens’ Vernacular 155: A group of rapists is a wolf pack while one working single-o is a lone wolf.
1990		(con. early 1950s) J. Ellroy L.A. Confidential 290: ‘Did you work closely with him?’ Stathis shook his head. ‘Jack was strictly the single-o type.’.
2001		‘Randy Everhard’ Tattoo of a Naked Lady 37: The very next night i was single-o and on the sneak.