Oliver was one of the first manufacturers to develop a model series that used as many interchangeable components as possible. In 1947, the model 88 was introduced above the 60 and 70 models, featuring a similar design to them. However, this was a short-lived intermediate phase.

In 1948, the Fleetline was introduced, which included the 4-cylinder 66 as well as the 6-cylinder 77 and 88. The appearance that distinguished it from competitors was primarily the fully sheet metal-covered engine compartment. The engines were produced in Waukesha. The lineup included HC models for gasoline, KD models for petrol, as well as later introduced diesel and propane versions. The diesels initially had technical problems but quickly replaced the petrol versions. All three models were available in Row Crop, Standard, Industrial, and Orchard versions. The Row Crop models could be equipped either with two front wheels placed in the center or with a wide, adjustable front axle, and their rear wheels were larger than those of the Standard models. Operator comfort was one of the most important design criteria for the new series. The noise level at the operator's seat was below 85 decibels, and the new Ridemaster seat suspension was complex. The power take-off was equipped with a separate clutch, making it one of the first tractors to have this feature. In 1950, the Hydra-Lectric was introduced as an optional feature — a hydraulic two-point hitch with electromagnetic working depth control. The first Fleetline tractors had band brakes that affected the drivetrain and included steering brakes. Disc brakes were introduced in 1951.

In 1952, the series was expanded with the similarly styled 99, which was, however, largely the same tractor as the earlier, unstyled 99.

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