In November 1891, notice was given of a private bill that would be presented to Parliament for the construction of the BS&WR. The railway was planned to run entirely underground from Baker Street to Waterloo. The route was approved and stations were permitted at Baker Street, Oxford Circus, Piccadilly Circus, Trafalgar Square, Embankment and Waterloo. Construction commenced in August 1898 under the direction of Sir Benjamin Baker, W.R. Galbraith and R.F. Church. By November 1899 the northbound tunnel reached Trafalgar Square and work on some of the station sites was started, but the collapse of the L&GFC in 1900 led to works gradually coming to a halt. When the UERL was constituted in April 1902, 50 per cent of the tunnelling and 25 per cent of the station work was completed. With funds in place, work restarted and works on the station buildings were under way. The additional stations were incorporated as work continued elsewhere and Oxford Circus was altered below ground following a Board of Trade inspection; at the end of 1905, the first test trains began running. The official opening of the Baker Street and Waterloo Railway (BS&WR, now the Bakerloo line) by Sir Edwin Cornwall took place on 10 March 1906.
The CLR and BSWR had separate surface buildings and lift shafts. The station buildings, which remain today as exits from the station, were built on very confined plots on either side of Argyll Street on the south side of Oxford Street, just east of the circus itself. The stations as originally built were entirely separate, but connecting passages were soon provided at platform level. The surviving Central London Railway building to the east of Argyll Street is the best surviving example of stations designed by Harry Bell Measures, and the Bakerloo line building to the west is a classic Leslie Green structure. Both are Grade II listed since 20 July 2011. Almost from the outset, overcrowding has been a constant problem, and there have been numerous improvements to the facilities and below-ground arrangements to deal with this. After much discussion between the then two separate operators, a major reconstruction began in 1912. This entailed a new ticket hall, serving both lines, being built in the basement of the Bakerloo station, with the Bakerloo lifts removed and new deep-level escalators opened down to the Bakerloo line level. The station has Cash machines, Euro cash machines, payphones, help points, wi-fi and escalators.
Connections: Victoria, Central Line. London Bus routes 3, 6, 7, 10, 12, 13, 23, 25, 55, 73, 88, 94, 98, 137, 139, 159, 189, 390, 453 and C2, and night routes N3, N7, N8, N13, N18, N55, N73, N98, N109, N113, N136, N137 and N207 serve the station. Additionally, bus routes 6, 10, 12, 23, 25, 88, 94, 139, 159, 189, 390, 453 and C2 provide a 24-hour bus service.