However that defence couldn't hold out fullback Monty Lama, crashing over near the posts.
First-five Peter Breen slotted the conversion as part of his stellar display.
The Northland squad member ran the cutter with precision, matching his prodigious boot with an incisive running game.
Breen kicked a penalty five minutes later after Wellsford went off their feet in the ruck to give OBM a 15-0 win inside the first 20 minutes.
Wellsford finally got an attacking chance after 20 minutes and they took full advantage of it.
They pounded away at the OBM line and prop Matt Talaese crossed over for a try.
Tyler Landsdowne added the extra two points to put them right back in the match.
Wellsford were reduced to 14 men shortly afterward as halfback Mark Talaese was sent to the bin for a deliberate knock-down.
OBM took the initiative with the extra man through loose forwards Nela Manu and Ethan Crutcher.
Firstly Breen put Manu through a hole 40 metres out to score, then Crutcher made a 45-metre break to score under the posts.
These tries gave OBM a 27-7 lead going into halftime.
A major highlight of the first half was the battle of the blindside flankers.
OBM's Aorangi Stokes was a monster on defence, putting on multiple crushing hits.
Meanwhile Conor Lawson was a menace with ball in hand for Wellsford, making plenty of breaks.
The second half contrasted the high-octane first half, with the game dissolving into an intermittent slog due to errors and penalties.
Wellsford looked to be back in the contest in the 52nd minute, shortly after OBM halfback Matt Marwick was shown a yellow card.
They capitalised on two penalties by crashing over through Landsdowne to put them within reach of OBM.
However, OBM scored a converted try as soon as Marwick returned through prop Phil Kite. A late yellow card to reserve flanker Joel Tugaga didn't cost OBM any points as they took the honours.
OBM will now face Mid-Northern at Toll Stadium in the final.
Mid-Northern qualified through a tight 25-20 win over Waipu.
