Ranging lock Justin Fredrickson busted through from 40m out to score with first-five Steve Grobler nailing the extras.
Otamatea struck back with a couple of tries of their own due to the impressive work of Solomon Alaimalo.
The centre first scored off a scrum 40m out, breaking the line and racing over.
He then played a pivotal role in the next try; drawing the defence in after another bust and putting winger Donald Pool under the posts untouched.
Both sides traded tries through Western Sharks centre Hamish Stokes and Otamatea's Matt Moulds to leave the game poised at 19-all going into the break.
The first ten minutes of the second half was a battle of attrition, both teams trying to gain the ascendancy.
Otamatea finally broke the deadlock with three tries in eight minutes of frenetic footy.
Second-five Jordan Hyland put Moulds over for his second try in the hooker's return to club rugby after being part of the Blues' Super Rugby squad.
Hyland then scored himself four minutes later after a lineout 5m out from the line.
A great kick deep into the Sharks' 22 saw them turn the ball over, giving bruising flanker Tevita Luani a try and Otamatea a 38-19 lead.
Staring down the barrel, Sharks gave themselves a glimpse when halfback Hamish Younger set up a try for fullback Tony Stokes after a good break.
However they couldn't cross again until the final 5 minutes, when the fullback scored a consolation try.
Otamatea coach Peter Robinson gave both sides props for a great spectacle.
"Western Sharks are a good side, we have to tip our hat to them," he said.
"Our boys put it all on the line and we're really happy with the effort."
Meanwhile, Western Sharks coach Mark Wells lamented his side's untimely errors.
"That lapse early in the second half really put us in a tough spot," he said.
"Having a few injuries didn't help us as well."
