 On 18 January 1931, the Bus Ring was established as an extension of Line 20 in both directions, so it ran from Frogner via Majorstuen, Sagene, Torshov, Rodeløkka, Grünerløkka, Kampen to Tøyen.In January 1934, Oslo Sporveier started building their own bus in cooperation with Strømmen.It featured hydraulic transmission, it lacked gears and a clutch, and had the motor under the floor.