It was the fifth time in the last ten years that O’Brien had participated in a deciding frame in the final qualifying round.
Liang Wenbo held on to deny veteran Fergal O’Brien a place in the 2020 World Snooker Championship – compiling a century break in the last frame to win 10-9 in Sheffield.
The Chinese led 5-2 at one stage but saw his fearless opponent fight back to retake the advantage.
When Liang fell 8-7 behind, it looked as though all of the momentum was with the Irishman.
But the 33 year-old, a former English Open champion, dug deep and constructed a marvellous 141 total clearance to clinch his Crucible berth – although questions have been raised over an undeclared foul by Liang in the deciding frame that could have potentially influenced the outcome.
While playing a safety off the pack of reds near the beginning of the frame, Liang clearly fouled by hitting a red with his cue, but the incident went unnoticed.
O’Brien was hoping to safeguard his place on the Main Tour, but the Dubliner will face an anxious wait.
Guaranteed to finish the season outside the top 64 in the world rankings, the 48 year-old must rely on staying inside the top four on a separate one-year list that will offer a backdoor avenue to tour survival.
Liang Wenbo, meanwhile, will be delighted to reach the venue stages of the World Championship for the first time since 2017.
Elsewhere on the first of two Judgement Days this year at the English Institute of Sport, young duo Alexander Ursenbacher and Ashley Carty emerged with hard-fought triumphs against more experienced campaigners.
The pair will make their debuts at the Crucible after respective 10-8 defeats of Andrew Higginson and Robert Milkins.
For Ursenbacher, who memorably just missed the chance to qualify as an amateur a few years ago, the 24 year-old will become the first Swiss player to feature in the last 32 in Sheffield.
As for Carty, by qualifying for the venue stages he is already assured of a fresh two-year card regardless of whether or not he finishes this term outside the traditional top 64 cut-off point.
The other matches on Monday were generally one-sided affairs, with Elliot Slessor a third player qualifying to make his first appearance at the Crucible with a comprehensive 10-3 success against Martin O’Donnell.
Three-time semi-finalist Alan McManus raced into a 6-1 lead against Louis Heathcote and never looked back, eventually recording a 10-5 win over the tour rookie.
Anthony Hamilton boasted a similar scoreline as the “Sheriff” denied Scott Donaldson to secure his long-awaited return to the Crucible for the first time since 2008, while Noppon Saengkham hammered Eden Sharav 10-2.
That wasn’t even the widest margin of victory of the day as Kurt Maflin destroyed Matthew Selt 10-1 with the help of some heavy scoring on the TV table.
The World Championship qualifiers will conclude on Tuesday with the remaining eight final-round fixtures to be played.
Two of the more intriguing battles see former world champion Graeme Dott in action against Martin Gould, and Ricky Walden taking on Matthew Stevens.