For Longest entry in the OED many years the verb to set has been cited as the longest entry in the OED. But a recheck shows that it has at last been toppled from this position. The longest entry in the revised matter is represented by the verb to make (published in June 2000). However, it is quite possible that set will regain its long-held position at the top of the league of long words when it comes itself to be revised.
In ranking order, the longest entries currently in the online Third Edition of the OED are: make (verb - revised), set (verb), run (verb), take (verb), go (verb), pre- (revised), non- (revised), over- (revised), stand (verb), red, and then point (the noun - revised).