The O-8 was a class of 25 Mikados, most of which had been rebuilt from other locomotives. In their final form they were the heaviest and most powerful locomotives of this wheel arrangement ever with a total weight of more than 750,000 pounds and a boiler output of 4,850 hp. The first three were built in 1932 in the Great Northern's shops when they had realized that their O-7 was not as powerful as they had hoped. The O-7 had been rebuilt from the class M to M-2 2-6-8-0 and had large cylinders, but their fireboxes were too small to develop enough steam at speed.

The three O-8, numbered 3397 to 3399, got a much bigger Belpaire firebox than the O-7 that included a combustion chamber and arch tubes. Additionally, the boiler pressure was raised to 250 psi and the superheater was made much bigger. Since the cylinders of the O-7 with a diameter of 31 inches were still too large, the O-8 got cylinders of 29 inches in diameter. This resulted in a weight of 367,000 pounds, an axle load of 70,000 pounds and a starting tractive effort of 82,900 pounds. 

Between 1944 and 1946, all 22 O-7 got new boilers from Baldwin and ALCO which had an even larger firebox area than the first three O-8. This was done by substituting the arch tubes against security circulators. The diameter of the cylinders was again reduced by an inch since it was still enough to create more than 77,000 pounds of starting tractive effort. They now weighed 425,540 pounds and had an axle load of 81,250 pounds. Numbers were still 3375 to 3396. Also the thee original O-8 were rebuilt in the same way. All were retired in 1957 and 1958.