The young pivot turned the south-western corner of his manicured, then muddied home track into his very own referee's corner to flip a scoreless game on its head.
Industrious tight forward Aidan Blair barged over from close range soon after, with May adding the extras with a finely judged conversion.
The visitors struck back with a penalty to Dion McNabb shortly after, but managed no more points as the hosts' superior line speed harried them into handling errors or misjudged kicks.
One such hack from the in-goal by McNabb essentially defined the contest.
The offending kick found its way to Otumoetai fullback Ryan McLean-Harvey, who did the rest in a fine style.
May converted from handy range, giving the home side a 14-3 half time lead.
The game degenerated as conditions worsened in the second forty, with only an unconverted, pick-and-go Rosehill try troubling the scorers.
Otumoetai captain Matt Ensor, whose rangy, physical presence caused plenty of problems in both the loose and the tight, applauded the approach of both teams given the conditions.
"It's the worst weather we have had all year and the first time we have been on telly so that's typical," he said. "The handling for the conditions was really good. Things haven't been going our way and we've lost games by two or three points, but to win the big one is good."
Otumoetai now chase a semifinal position in the Bay of Plenty secondary school competition.
Ensor said the team likely needs two wins from its final three matches to reach the post-season.
Otumoetai College 14 (Aidan Blair, Ryan McLean-Harvey tries; Jack May 2 cons) Rosehill College 8 (Chavez Zachan try; Dion McNabb pen)
