Green’s Dictionary of Slang

1. (US tramp) a tramp who travels alone.

1931		Irwin Amer. Tramp and Und. Sl. 170: Single O. – [...] One travelling alone by preference.
1949		Monteleone Criminal Sl. (rev. edn).

2. (US Und.) a criminal who works alone or the crimes thus perpetrated.

1930		R. Chadwick in Liberty 5 July 20/2: The first single-o is a street heist.
1931		Irwin Amer. Tramp and Und. Sl. 170: Single O. – One working a lone ‘game’ or ‘racket’.
1949		Monteleone Criminal Sl. (rev. edn) 168: on the single o A pickpocket working without a confederate.
1967		M. Braly On the Yard (2002) 231: He might have done all right if he had stayed on the single-o, but [...] he wasn’t a true solitary.