The Pennsylvania Railroad's class O1 comprised eight experimental boxcab electric locomotives[4] built in 1930 and 1931.

For production, the PRR chose to concentrate on the P5 class, effectively an enlarged and more powerful version of the O1 with an additional pair of driving wheels.

[6] Each sub-class was fitted with a different combination of traction motor power output and drive gear ratio.

In addition, three O1 locomotives were fitted with General Electric equipment, three with Westinghouse, and two with Brown Boveri.

Later in life, they were used for transfer work around Penn Station and Sunnyside Yard, mostly hauling empty passenger stock.