The class O was a 2-8-0 locomotive for freight service on light rails around the Eastern Goldfields. It had small drivers with a diameter of three feet and cylinders with a diameter of 15.5 inches and a stroke of 21 inches. A special feature were small additional water tanks on the sides of the boiler. 36 were built by Neilson between 1896 and 1898, while a further ten came from Dübs. It took only a few years until they were replaced on the mainline by the class Ec mixed-traffic Pacific and the class K 2-8-4T. From now on, they were used on branch lines. In 1907 and 1908, ten were rebuilt to class N 4-4-4T suburban tanks.

As it turned out that the others needed larger coal supplies for light agricultural work, the bunkers were enlarged from three and a half to seven and a half tons. At the same time, the water tanks were reduced from a total of 2,500 to 2,000 gallons. Remains of the ten which had been rebuilt to class N were used to create ten new locomotives, which received drivers of three feet and six and a half inches and also had the larger coal bunkers. These were designated class Oa. All were withdrawn between 1940 and 1962. Only O218 was preserved and is being owned by the museum in Bassendean. From 2015 it was on a loan to the museum in Walkaway.