OBM were terrific against YMP in the slippery conditions, sometimes making it look easy on attack and showing heart on defence.
Adrian Wyrill ran rampant at openside flanker, Gabe Te Kani had impressive moments at No.6 and Mark Atkins was again a handful at second five-eighth.
They each scored two tries and reinforced their claims for Poverty Bay Heartland selection this year.
Centre John Jones kicked seven out of seven conversions — a remarkable feat in the conditions.
Halfback Willy Grogan threw himself into his defensive duties and OBM coach Trevor Crosby said Jake Holmes stood up at No.10, directing the team’s attack.
Crosby said he was pleased with the unity his players showed on the field.
“It was a tough game but we created more chances and we took them,” he said.
Despite the conditions, “we still moved the ball around, which was good”.
YMP coach Steve Smith suggested it might not be a bad thing that his team had to play again next week.
“I’m thankful we get to fight another day.”
Smith said OBM were well organised and his own team “hung in there”.
A few players looked short of a gallop, though, and YMP are running out of time to recapture the fizz they put into top-notch team performances earlier in the season.
Smith said they were up against it with OBM’s mobile forward pack, and his team struggled to put phases together, but he was happy with his substitutes bench.
OBM piled on the points steadily — Wyrill scored his first try after five minutes, Atkins dotted down after about 20 minutes, Wyrill scored again after 25 and Te Kani scored after about 32.
YMP were pinned in their own half for much of the first 40.
They tightened things up after halftime, keeping the ball in the forwards for a period on attack.
But OBM broke out with a try to wing Keelan Poi and then Te Kani scored his second.
It didn’t help YMP that their opponents were going up in multiples of seven, thanks to Jones having a perfect day with the boot off the kicking tee.
YMP lock Nehe Papuni and No.8 Jesse Kapene scored tries in the second half but Atkins also dived in for his second try.
Waikohu got out to a handy lead in their game against HSOB, who threatened to reel it in during the second half and closed the gap to four points before the 2018 champions regained control.
Waikohu replacement forward Sonny Campbell scored a crucial try with 10 minutes left to play, putting the game almost out of reach as rain tumbled down.
Waikohu started the game with Lane Reeves at first five-eighth, and Kelvin Smith and Ethine Reeves as the midfield pairing. Jesse Fleming, playing in his first game since Round 1, started on the wing. They exerted control in the first half.
The teams traded penalties but Smith scored the first try and loose forward Lynden Manuel extended the lead for Waikohu with a try just before the break.
The Waikohu bench was light on numbers and prop Sandy Hohipa-Campbell left the field with a back injury.
HSOB had opportunities but attacking players ran into their own men more than once.
Captain and No.8 Tamanui Hill was also ruled to have been bundled into touch when he reached out to score.
HSOB struggled to get into the contest but looked a different team after the break.
Their attack became fluent and a turnover snaffled by outstanding openside flanker Stefan Destounis led to first-five Baxter Mackay running down the right sideline and in for a try.
A scoreline of 20-13 became 20-16 when fullback Andrew Tauatevalu slotted his third penalty goal with 28 minutes left to play.
But Waikohu weren’t going to surrender and they met the challenge from HSOB with stiff resistance.
Waikohu coach Jason Tuapawa said his side had been prepared for a comeback and they tried to starve their opponents of the ball.
HSOB coach Scott Leighton said he was disappointed with the loss.
“We didn’t do the basics well,” he said.
Their season started with knocking over Waikohu but they had a 50 percent win record before their knockout semifinal.
HSOB beat every team during the season, except for OBM, but also lost against every team, except Pirates-GMC.
Waikohu started the season slowly but have got better in the second half of their campaign.
Crosby said the winner of next week’s game — YMP or Waikohu — would be tough to play in the final.
He also said use of the scoreboard, lights and a ground announcer helped to produce a good occasion on Saturday.
Semifinal, 1 v 2:
OBM 49 (Mark Atkins 2, Adrian Wyrill 2, Gabe Te Kani 2, Keelan Poi tries; John Jones 7 con) YMP 10 (Nehe Papuni, Jesse Kapene tries). HT: 28-0.
Knockout semifinal, 3 v 4:
Waikohu 27 (Kelvin Smith, Lynden Manuel, Sonny Campbell tries; Smith 2 pen, 3 con) High School Old Boys 16 (Baxter Mackay try; Andrew Tauatevalu 3 pen, 1 con). HT: 20-3.