The 50-year-old Irishman joined Rod Lawler, Andy Lee and Bai Langning in securing a place on the professional circuit for the seasons 2022/23 and 2023/24.
The former world number nine and British Open winner was relegated from the tour at the end of last season but bounced straight back with a 4-1 win over Rory McLeod.
"It's a tough week," he told World Snooker Tour.
"If you don't get through the first event here then the pressure is only going to build.
"I'm really proud of the way I battled for everything and kept a good attitude.
"A couple of weeks ago I wasn't even sure whether I would play in Q School. If I was going to play then I was going to give it everything."
O'Brien revealed he would be more selective in the competitions he chooses to play in over the next two years so that he has a better work-life balance.
He said: "I don't see myself playing more than two more years and I plan to play a limited schedule.
"Over the last couple of years I've been chasing points and haven't always enjoyed it. That frees me up to choose the tournaments I want to play and take more breaks."
The former International Open finalist Lawler let slip a 3-1 lead against Brandon Sargeant in the final round but came through the decider 4-3 to return to the professional tour.
"I'm really delighted to be back on tour," he said.
"I can't give up the enjoyment of the battle, that's what I have really missed over the past year. I'm coming towards the end of my career so I have to embrace the next two years and enjoy it."
Lee of Hong Kong will return for his third time after a two-year absence following a 4-2 win over Luke Simmonds and the 41-year-old said the make-or-break nature of Q School had him on edge.
"It's disbelief, I'm shocked to have got through," said Lee.
"When Covid hit in 2020 I went back to Hong Kong and I couldn't come back to the UK. So I haven't been playing to any kind of elite level over the past two years. I have practised occasionally with Marco Fu which has really helped.
"The last round of Q School is the worst match that any snooker player can be involved in. There is so much at stake, it's all or nothing."