OBM coach Greg Shipton said despite the final score, his side was made to work hard for the result.
"The score didn't really indicate how the game went really. We ended up getting away from them in the last 20 minutes or so, but it was a tough old game," he said.
"Both sides played well but we toughed it out and got home in the end.
"The big key in the second half was keeping possession. We were playing into the wind so we wanted to hold on to the ball and not kick it away. We did that pretty well and it showed at the end.
"I thought our backs were outstanding all game and while our forward group is a bit smaller than the Mid Northern guys, they grunted away in the pack."
OBM crossed for six tries through Jarod Byrnes (two), Noah Cooper, Monty Lama, Ethan Crutcher and Renata Te Hana. Te Hana added three conversions while Jayd Kapa also nailed a pair.
Lock Thomas Dwyer and wing Ratu Mara scored for the visiting Mid Northern, with Weylan Skips added the extras and a penalty goal.
Mid Northern coach Craig Edwards said his side were not clinical enough to get the result they wanted.
"Obviously we're disappointed with the end result. I thought we had a chance coming into the second half but it wasn't to be," he said.
"Their backs outplayed us and our ones missed too many tackles and made too many errors.
"Our forwards were strong though, they pushed the OBM pack around but we made too many mistakes.
"Our forwards are a big strength of ours and we have a good backline, but they just didn't click on the day. Making the top four is what we're looking at this season."
The other matches on opening day threw up some exciting action.
Hora Hora started their 2017 campaign with a 17-0 shut out win against Mid Western, while Hikurangi took out the closest match of the round with a 25-17 win over Kamo.
Waipu started their season strongly with a 27-10 victory against Wellsford, while Western Sharks showed they will be tough to beat this year with a 54-12 drubbing of Otamatea.
The reserve grade also provided excitement for the rugby faithful.
Mid Northern had a clinical 34-7 win against defending champions OBM, while Hora Hora demolished Mid Western 46-3.
The other two matches were tight tussles with Western Sharks beating Otamatea 24-15 and Waipu outlasting Wellsford 19-15.
Bayleys' Premiership results:
Old Boys Marist 40 Mid Northern 17
Hora Hora 17 Mid Western 0
Kamo 17 Hikurangi 25
Otamatea 12 Western Sharks 54
Wellsford 10 Waipu 27
Reserves:
Mid Northern 34 Old Boys Marist 7
Hora Hora 46 Mid Western 3
Otamatea 15 Western Sharks 24
Wellsford 15 Waipu 19
