Onslow won the toss and chose to bowl.
Hard-hitting left-hander Tom Rouse topscored for Poverty Bay with 13 off 26 balls, including three boundaries, as his side were dismissed for 99 in 25.2 overs.
Paceman Thomas Cooper (five wickets for seven runs off 2.2 overs) was Onslow's Most Valuable Player.
Taylor Scott (10 from 28 balls in 47 minutes at the crease at No.7) earned the Bay's MVP award for grit, bat in hand, and an excellent effort with the ball.
Seamer Scott the seamer took 2-9 off four overs.
He bowled five dot balls to begin a 30th and final over at the start of which Onslow needed one run to tie, two to win.
They got there on the last ball to be 100-6.
The Bay fought hard, Vincent Patrick-Page's 2-9 off two overs proof of their improvement with the ball in a nail-biter.
Onslow coach Rhys Morgan praised the Nathaniel Fearnley-led side as “an outstanding bowling and fielding unit”.
A slick Collegians outfit showed what a team who have been together for six years can do.
Collegians won the toss and amassed 233-2, No.5 Fred Oppenhuis (50 not out) racking up his second unbeaten half-century of the day.
First-drop and team MVP Gethyn Healy also retired at 50, No.4 Oliver Harland made 36 and opener Charlie Jackson 35.
Jackson was caught and bowled by Poverty Bay Chinaman bowler Riker Rolls (1-25 off three overs). Fearnley (1-18 off 4) was the other wicket-taker for Poverty Bay.
Healy (3-3 off 2) and Samuel Dowell (2-4 off 3) spearheaded the strong Collegians pace attack as Poverty Bay were 72 all-out in 18.3 overs,
No.5 Fearnley was the Bay's leading scorer with 29 off 21.
Collegians coach Andrew Crombie complimented Fearnley and Rolls on bowling an excellent line.
The Poverty Bay captain was also positive with the bat, he said.
Poverty Bay today play 30-over matches against Taupo and Taranaki.
