Green’s Dictionary of Slang

(UK Und./police/prison) police observation; prison observation room.

1961		J. Sparks Burglar to the Nobility 127: She kept obbo on the corridor while I climbed in through the window.
1970		‘Metropolitan Police Sl.’ in P. Laurie Scotland Yard (1972) 325: obbo: an observation, which is ‘kept’ not ‘made’.
1989		J. Morton Lowspeak.
2003		Observer Crime 27 Apr. 28: Obbo. Police observation on criminals.
2004		Guardian G2 4 Mar. 18: But guv, there’s some poor geezer on the deck in the obbo.
2007		N. ‘Razor’ Smith Raiders 288: They didn’t know that Old Bill had had them under obbo for quite a while.
2016		Skepta ‘Corn on the Curb’ 🎵 I've got goons that can't leave the country / Above the law, under obbo.