Adrian Bromage in 1998 was the last Shark to win the award before O’Brien, who beat out the favourite, East Perth’s Paul Johnson, by 12 votes.
Ineligibility did not stop teammate Mark McGough from polling fourth.
East Fremantle chief executive Adam Kelly said the club took ‘great delight’ in O’Brien’s win.
‘Rory has been at our club for three years and from the moment he arrived he has added significant value to the culture of our club and at all times presented himself professionally,’ he said.
‘He thoroughly deserves this accolade and we think it is a just reward for his dedication and commitment and the support offered by his wife Ebony.
‘Rory sets an outstanding example to his team mates and is respected by all at the club.’