And when Waikohu launched counter-attacks, OBM fullback and skipper Jake Holmes kicked superbly to push them back on defence.
Three times OBM went across the line, only to be held up. But eventually the pressure told.
One minute before halftime, assistant coach Keith Henderson, playing at first-five, reduced the deficit with a try following another forward drive.
Seven minutes into the second half, OBM halfback Willy Grogan, who was like an extra loose forward in the driving rain, levelled the scores with an opportunist try.
It came after the OBM pack drove across the line but yet again failed to get the ball down. From the base of the resulting scrum, Grogan picked up the ball and powered his way through two defenders to score to make it 10-all.
At this stage, only one side, OBM, looked likely to win this match. They continued to play 10-man rugby and had Waikohu under all sorts of pressure.
But Waikohu, the only unbeaten side this season, showed their class and character by reclaiming the lead against the run of play in the 57th minute when, after the hint of a forward pass, Wilson bagged his second try.
Undaunted, OBM went straight back on attack, but when Henderson was sinbinned in the 65th minute, Waikohu made OBM pay dearly with two tries in two minutes.
Prop Sandy Hohipa-Campbell’s five-pointer, converted by first five-eighth Kelvin Smith, took them past the seven-point margin — 22-10.
Right wing Larry Fleming then stretched the gap to 17 points with his first try of the season.
In the 75th minute, Henderson made some amends for his sinbinning with his second try, but it was too little, too late.
Despite the yellow card, Henderson’s experience and direction on the field brought a lot of stability to a young side in which Grogan, Cook and Otene-Ria were outstanding.
Geoff Pari, standing in as Waikohu coach in the absence of Jason Tuapawa (family commitments), said OBM got their tactics “bang-on”.
“We couldn’t get our hands on the ball in the first half,” Pari said.
“All I said to the boys at the break was to play smarter, play to the conditions, and if there’s one thing about this team, it’s that they never give up.”
On a day when the much-vaunted Waikohu backs were starved of possession, Hohipa-Campbell was named player of the day, followed by fullback Ethine Reeves, who looked dangerous whenever he was in possession, and No.8 Ken Houkamau, who led the tackle count.
Waikohu will take a five-point lead into the second round of the championship.
Their bonus-point victory in the final game of the first round took them to 30 points, five ahead of Ngatapa, who thumped Tapuae 54-0 at Patutahi.
YMP, who had the bye, are third on 20 points, followed by OBM on 14.
High School Old Boys, who beat Athletic 17-14 at the Oval, are next on nine points, ahead of Athletic, five, and Tapuae, yet to pick up a point.
WAIKOHU 27 ( KC Wilson 2, T Noanoa, S Hohipa-Campbell, L Fleming tries; K Smith con).
OBM 17 (K Henderson 2, W Grogan tries; J Holmes con).
HT: 10-5 (Waikohu).