Green’s Dictionary of Slang

1. obligations.

1923

J. Manchon Le Slang.

2. a lookout; an act of (police) observation; usu. in phr. keep obs, to keep a lookout [abbr. SE observation].

1962		R. Cook Crust on its Uppers 45: I had the obbs on you.
2000		J.J. Connolly Layer Cake 52: The old man’s little obs team has tipped ’em the wink.
2011		J.J. Connolly Viva La Madness 61: Unless they let you run [...] let you go wandering but tailing you in, under obs.

3. (Aus./US prison, also obso) an observation wing or cell of a prison [abbr. SE observation].

1950		Goldin et al. DAUL 148/1: Obso. (P) A cell or cells reserved for the observation of recalcitrants suspected of mental deficiency.
1967		K. Tennant Tell Morning This 285: He was given a week’s special hards, and told he was an ‘O.B.S. job’, which frightened him.
1990		Tupper & Wortley Aus. Prison Sl. Gloss. 🌐 O.B.S. Contraction of observation wing or place for emotionally disturbed prisoners. Hence an O.B.S. case is someone viewed as crazy.
1999		D. Looser ‘Boob Jargon’ in NZEJ 13 34: OBs n. Observation Cell- ashipcell with acamera.
2001		D. Looser Boobslang [U. Canterbury D.Phil. thesis] 128/1: obs cell or. obs room) = observation cell.