To accommodate increased mail traffic, the meter-gauge Federated Malay State Railways ordered class O Pacifics from North British at the end of the 1930s. Due to individual batches built between 1938 and 1946, they were given the designations O1 to O4. They were built to achieve the greatest possible power with a permitted axle load of 11 3/4 tons.
These were the most advanced Pacifics in the FMSR. Lightweight construction was used in many places so that the boiler and engine could still be sufficiently dimensioned. The boiler pressure was chosen to be relatively high at 250 psi to increase power. Three cylinders were also used, which were enlarged by half an inch in the third and fourth series. Other special features were thermic syphons and a rotating valve gear.
They were successfully tested for possible high-speed traffic at 70 mph or 113 km/h, but were not actually used. Although these locomotives were later converted to run on oil, they were soon replaced by diesel locomotives in the fast mail service. They therefore continued to be used in freight transport for a long time.