That fast start was slowly eroded as the wind picked up and OBM kept the territory game firmly in their opponents' half.
Referee Aaron Brown set the tone early and was not taking any grief as the teams tussled for the upper hand. Brown penalised both teams often. The game cleaned up as players realised he wasn't going to take nonsense.
A mistake by Pirates fullback Rico Walters gave OBM the opening they needed to score the first try of the game.
Walters returned a deep range-finding kick with a high bomb, and his run to the falling ball left no one covering the backline.
Strong forward ball gave OBM halfback Nicolas Carrizo space to box-kick behind the Pirates line but the work wasn't done for OBM.
They kicked the ball on and with several more support runners in their flanks it was ultimately an easy try for centre Hunter Tangira, who dotted down for a converted try.
The pressure on Pirates continued. A free-kick at the scrum for not pushing straight, and a quick tap led to the ball being held up on the line after a series of pick-and-goes.
Pirates were strong in the resulting scrum, and OBM were shunted back and forced into a mistake.
The ball squirted out the back of the scrum and was kicked on by Pirates, who piled into the breakdown but were penalised for going off their feet.
OBM second five-eighth Rikki Terekia then showed why he normally plays in the forwards. A quick tap from 15 metres out as the Pirates retreated gave him the front foot and a head of steam that couldn't be stopped. In a storming run, he shrugged off multiple defenders, before carrying several more over the line on his back to score. OBM led 12-3 at the break and it was anyone's game.
Pirates opened the scoring with winger Rasari Solomani hitting a gap in the defence and making a strong run down the right-hand side of the pitch, scoring a try in contact. The try was converted from the sideline with an excellent kick from Jacob Leaf, who kicked well for Pirates all game.
There was a bit of niggle between the two teams after Juston Allen was yellow-carded for a high tackle and swinging arm. Pirates capitalised on OBM being a man down.
A strong scrum gave Pirates a good base for a hit-up from first receiver before Leaf wrapped around the ruck, running on to the ball with pace and scoring a strong try as both forward packs were preoccupied with a tussle in back play. The try was converted, giving Pirates the lead.
OBM had several strong attacks, but they came to nothing when players were penalised in the opposition 22, which gave Pirates the break they needed to clear the ball.
OBM coach Trevor Crosby said he was disappointed with some of the calls.
The pace of the game was telling on Pirates, some of whom were on their haunches and lying down during breaks in play.
Two successive penalty kicks gave Pirates a cushion. With seven minutes to go and 11 points the difference, the game looked out of OBM's reach.
They weren't going to give up, though, and worked the ball back into the Pirates half.
After a series of pick-and-goes close to the touchline, second-five Terekia crashed over for his second try, showing again why he normally plays front row.
The kick wasn't converted and OBM were still down by six.
Then, quickly off the restart, OBM worked the ball to the right-hand side of the pitch. Fullback Michael Fox found himself in space on the blindside, and made a darting run through a gap as defenders closed in.
He looked like he would be forced out of play, but a miracle offload in the tackle enabled the unmarked Hunter Tangira to run the ball in for a try close to the posts.
Fox converted to give OBM a one-point lead with one play remaining.
OBM held on to the ball, and finished the game strongly.
Crosby said he was proud of his team, who were not only playing for themselves, but also for the Gilroy family who had lost a family member during the week.
“They mean a lot to OBM . . . I'm a proud coach, not only for OBM but also for the Gilroy family.”
Pirates coach Willie Waitoa said they lost some of their key playmakers in the last 15 minutes of the game, including skipper Willie Bolingford, who came into the game with an ankle injury.
“We were our own worst enemies,” Waitoa said.
“I thought it was a 20-point wind.”
Six of the named starters were not present for Pirates, and Waitoa said it was good to see young players getting experience on the park.
“It was an opportunity to get a few more minutes for our younger players, not just for next weekend but also for next year.”
OBM 24 (Hunter Tangira 2 ,Rikki Terekia 2 tries; Michael Fox 2 conversions).
Pirates 23 (Rasari Solomani, Jacob Leaf tries; Leaf 2 conversions, 3 penalties).
HT: 12-3 (OBM).