The 33-year-old won his fourth tournament of 2022 after beating his Irish opponent 8-2 to round-off an exciting weekend of darts in Budapest.
The 2022 Masters champion and Premier League runner-up scooped the £25,000 top prize, claiming European Tour glory for the first time since the 2020 International Darts Open.
Earlier in the night, O'Connor came from 5-3 down to edge out Michael Smith in Sunday afternoon's third round, before defeating Jose de Sousa and Nathan Aspinall to reach his first European Tour final since 2018.
Cullen, on the other hand, had battled past Martin Lukeman in Saturday's second round, and saw off Jim Williams in Sunday afternoon's third round to continue his challenge.
He then overcame Rusty-Jake Rodriguez 6-4 in the quarter-finals and survived three missed match darts from Dave Chisnall during their semi-final, before producing a 12-darter to complete a 7-6 victory.
Cullen's most dominant display came in the final, as he claimed the opening five legs without reply, twice punishing missed doubles from O'Connor whilst also posting a 13-dart leg, two 14-darters and a 100 finish.
O'Connor hit back with double ten and then produced a 124 finish to break with a sparkling 12-darter, but five further missed doubles in the next leg allowed Cullen in to move 6-2 up.
The World Series star then edged towards victory with an 11-darter as he took out 96, and four further misses from the former World Cup finalist allowed him to claim the title on double eight.