O is a series of electric locomotives used by Swedish State Railways (Swedish: Statens Järnvägar, SJ) for hauling heavy iron ore trains on the Iron Ore Line in Sweden.

The O series was built for the first electrified railway in Sweden, Malmbanan.

ASEA started testing out technology with a test unit in 1908, and by 1914 the first Oa locomotive was delivered, along with the passenger hauling Pa. At first it was the northern part of Malmbanan, from Kiruna to Narvik that was electrified.

The steam locomotives had been hauling 28 wagons at 10 km/h, but the new agreement signed in 1913 with LKAB meant that SJ had to haul 40 wagons at 30 km/h to transport enough ore.

The solution lay in a joint production between Siemens of Germany and ASEA.