Captain Adam Sylva (2-17) was the pick of the bowlers as they restricted Kamo High to 112/5.
Onerahi started in shocking fashion as they lost their first three wickets for just 16 runs through a combination of good bowling from Connor Bolton (1-23) and some poor running.
However, Kirk Henderson came in and, combining with useful contributions from the middle order, saw Onerahi home with an exuberant 59 which included nine boundaries.
This set them up for a clash with City Reserves, who had beaten Bream Bay by seven wickets in the first game on the back of 43 from Harry Sekhon.
Onerahi lost the toss and were sent in to bat and unlike their first match of the day, started strong with contributions from the entire top order.
BJ Howie led the way with 31 and was supported well by Sylva (17), Henderson (18) and Ryan Woodman (20).
Onerahi lost momentum in the final overs as Kieran Nelson (2-21) and Reagan Radford (3-14) limited them to a decent total of 134/8.
City's reply was shaken early by the opening bowling of Axel Sheppard (4-27) and Callum Hartnett (1-14) as they collapsed to be 40-6.
Sheppard also provided a moment of brilliance as he ran out Stephen James with a return direct hit after James was too casual about getting back into his crease.
Lower order contributions from Radford (18), Tim King (17) and Jack Ingram (13 not out) brought them close and gave them a chance of victory.
However some standout death bowling from Martin Tansley (3-17) ended City's innings 16 runs short.
Bream Bay comfortably accounted for WBHS after bowling first.
WBHS struggled their way to 95/8 off their 20 overs as the Bream Bay bowling attack restricted their scoring zones and took wickets consistently throughout the innings.
Quinn Pooley top scored with 40 not out but found support hard to come by.
In reply Bream Bay cantered home to a 10 wicket win, knocking off the total in 14 overs with openers Manjot Singh and Satta Singh carrying their bats through the innings.
The final between Bream Bay and Onerahi Reserves will take place at Cobham Oval on December 11.