Green’s Dictionary of Slang

the London Underground.

1960		J. Franklyn Dict. of Rhy. Sl.
1975		‘P.B. Yuill’ Hazell and the Three-card Trick (1977) 5: It was down there in the Oxo cube that Mr Spencer had hit the rails.
1981		P. Wright Cockney Dialect and Sl. 103: oxo cube. ‘tube, London underground’.
2000		J.J. Connolly Layer Cake 95: You can [...] jump on the Oxo cube, come here and spend the afternoon ironing it out, getting pampered and rubbing shoulders with peers of the realm.