O'Brien has been on the professional circuit for 30 years, but after suffering an early defeat in this year's world championship qualifiers, the Irishman lost his place on the tour.
The 1999 British Open champion needs to reach the Q School semi-final in order to secure his status as a professional player. As things stand, he is just three wins away from doing just that.
O'Brien took on Martin O'Donnell in the third round and put on a solid performance to clinch a 4-2 victory.
Breaks of 61 and 62 helped the 50-year-old to a 3-1 lead before his opponent, a former UK Championship quarter-finalist, narrowed the gap with his own impressive break of 71.
Nevertheless, the former world number nine finished strong and won the last frame with a score of 73 to seal a fourth-round clash against Jenson Kendrick.
Elsewhere at Q School
China's Liu Hongyu, who currently holds the highest break at this year's Q School (137), eased past Alfie Lee to secure his place in the next round.
The 18-year-old won 4-1 and produced more eye-catching snooker with breaks of 100 and 71.
Liu will now take on Germany's Lukas Kleckers who is looking for an immediate return to the professional circuit.
The 26-year-old picked up a 4-1 win over James Cahill, producing breaks of 75 and 61 in the process.
Cahill, just like O'Donnell, will turn his attention to the two remaining Q School events as he looks to avoid a second successive campaign as an amateur.
Lee Walker will make an appearance in the fourth-round following his 4-2 defeat of Ashley Carty.
Rory McLeod will join him following a 4-1 victory over Hamim Hussain.