The 1-Series model use a total of 4 O2 sensors Two before the catalyst and two after the catalyst. Select O2 sensor location from the menu. Made by BOSCH. OEM. Sold each.
Part numbers below are for reference.
Part # 11 78 7 558 055 Pre Cat in Front Manifold. Cyl 1-3 (1400mm length)
Part # 11 78 7 558 073 Pre Cat in Rear Manifold. Cyl 4-6 (910mm length)