Jordan Kose and Shannon Kloppers piled the pressure on the home side as they set a great platform for the visitors.
Onerahi's fielding let them down as they put down six catching chances - and the batsman they gave lives to took full toll.
Kose (98) and Kloppers (69) provided the bulk of the runs as Rodney raced towards the 300-run mark.
However, Onerahi showed fight and Robert Lee (3 for 54) picked up a trio of scalps to somewhat halt Rodney's charge.
A late collapse stopped Rodney short of 300 but 265 for 9 was nothing to baulk at.
Onerahi knew they needed a hot start in order to haul in the total and that's exactly what they got as openers BJ Howie and Stephen Guy quickly set about their work.
A great opening stand of 135 between captain Howie (67) and Guy (56) was promptly followed by middle-order struggles as they couldn't find their feet, eventually being dismissed for 195, falling 70 runs short of the Rams' total.
Pieter Lambrechts provided a little bit of fight in the middle order being eventually run out on 24. Ultimately the effective bowling of Ben Curtin, who snared 5 for 43, and Sam Beehre (3 for 25) proved too much.
"Defeat grasped from the jaws of victory," said Greg Guy, the person the shield is named after.
It was a disappointing result for Greg Guy's old club as fielding woes and a top-class effort from Rodney denied Onerahi the spoils.
