O'Brien came from 2-0, 4-3 and later 7-6 down to win 9-7 as he picked up the trophy and a cheque for pounds 60,000. The additional bonus for him is a place in the world top 16 for the first time as the circuit prepares to move on to Sheffield for the Embassy World Championship, starting next Saturday.
O'Brien knocked out the defending champion, John Higgins, in the last four and he followed that up with this historic win over Hamilton, who was also appearing in his first major final since joining the paid ranks.
Hamilton, the world No 11 from Nottingham, did not disgrace himself; in fact, he repeatedly outscored O'Brien but the Dubliner held his nerve when it mattered as he became only the third Republic of Ireland professional - after Ken Doherty and Patsy Fagan - to collect one of the game's major trophies.
Hamilton opened the match with back-to-back breaks of 110 and 134, while his third century of a high-quality contest - 121 - put him back in the driving seat at 7-6 in front with a possible four frames to play. But it was O'Brien who finished on top, taking three frames without reply, including a break of 129 in the 15th frame.
The end for Hamilton came as he attempted to clear up to force a deciding frame. He missed the final blue into a middle pocket but seemed to have left the pink safe on a side cushion. However, after potting a straight- forward blue, O'Brien doubled the pink into a middle bag and sank the black to succeed Higgins as champion.
While O'Brien remained calm, the moment of victory was too much for his wife, Jean, who broke down in tears as she sat alongside his father, Fergus, and mother, Joan.