Horouta had an upset six-run win over HSOB, who won the then-T20 Walker Shield against Horouta by 63 runs in last season's final.
On Thursday, Horouta captain David Situ (61) and Gautam Sareen (42) shared an opening stand of 102 as Te Waka reached 120-4.
Off-spinner Anthony Boyder (2-22 in three overs) was the blue and whites' best bowler but it was Te Waka's day.
Despite the best efforts of opener Steve Lamb (28) and first-drop Graham Sharp (29), Sareen took 3-21 off three overs and fellow medium-pacer Billy Morse 2-9 from three overs, with one maiden, as Horouta restricted HSOB to 114-8.
HSOB needed 13 runs to win at the start of the 15th over.
Horouta conceded seven in a thrilling finish but Sareen bowled Taye McGuinness (4) with the last ball of the match.
The all-round heroics of Sareen, Morse's ability to bowl line and length under pressure and Situ's dedication with the bat could be needed tomorrow.
OBR may send out left-handers Te-Reimana Gray and Sean Henry to open the batting, while the Stewart brothers — full-of-length medium-pacer Paul and off-spinner Daniel — give them good bowling options alongside Jimmy Holden, Matt Cook, Richie Needham and Nick Armour.
Dane Thompson's return adds firepower while the hitherto injured Hayden Keast is also a possibility to play.
Keast can still get the ball through and is a more refined operator than the unpredictable terror who for Gisborne Boys’ High once had OBM's Lloyd Gabriel hopping from foot to foot courtesy of a "sandshoe crusher" that hit his toe.
Tomorrow's Doleman Cup semifinal is set to start at midday under the auspices of umpiring stalwarts Jason Trowill and Stewart Patrick.
In Senior B Grade action — Round 7 of the Hope Cup on Nelson Park's artificial pitches — defending champions Civil Project Solutions Ngatapa Green Caps host the Campion College first 11 (pitch 1), Breakers Horouta Te Waka face Rawhiti Legal Old Boys' Rugby (pitch 3) and Chicking High School Old Boys' Presidents play Gisborne Boys' High School second 11 (pitch 4).
All three Senior B games start at 2pm.