I think the following appeared at the front of APBA's OPC rule book for a decade or more:

By Edgar Rose

OPC, the Outboard Performance Category of APBA, was the outgrowth of boat racing
enthusiasts' desire in the 50's and early 60's to compete in their family boats with standard mass produced outboard motors larger than the 40 cubic inch motors racing in the APBA Stock Outboard Category. This desire led in 1961 to the hugely successful race on Lake Tohopekaliga in Kissimmee, Florida, run under APBA's Special Events, and organized by E. C. Kiekhaefer, the legendary head of Mercury Outboards, under rules prepared by Charlie Strang, his right hand man at Mercury, and Jimmy Jost. This 6-hour marathon had 62 entries and was won by Jon Culver and co-driver Chuck Merserau.

In 1962 Jon Culver was appointed Chairman of the newly formed OPC Category, with Edgar Rose as Chairman of the Technical Committee in charge of the rapidly evolving rules. Though OPC was initially called "Outboard Pleasure Craft" the name was changed a number of years later to "Outboard Performance Craft". The first OPC National Championships were held in 1963 at Lake George, NY and consisted of Closed Course Nationals, Marathon Nationals, and Kilo Trials. National Championships have been held annually ever since, with more than 200 entries at some of them.

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A tip of the hat to Claude Fox and NOA ... NOA had a pleasure boat division and formal rules in place sometime before 1958. The 1959 NOA rule book has some speed records listed as being set in 1957. That was probably the first year.

Another forum has the original Albany to NYC race incorrectly listed as an early OPC race; actually it was an early STOCK outboard race. Stock Outboard's first few years greatly resembled OPC, but evolved into something completely different.

To my knowledge, OPC never had A or B classes.