The 36-year-old announced after landing his fourth Crucible title that he intended to take a lengthy break.
It later emerged that the unavailability of appearance money was a factor in his refusal to sign a World Snooker players' contract, which he said was "too onerous".
He has missed ranking tournaments in China and Australia.
Tuesday, at midday, was the deadline for O'Sullivan to sign and be eligible for December's UK Championship in York.
His management company Grove announced on Twitter: "Good news. @ronnieo147 may be playing again. Contract clarified. Uk and international in china entered."
O'Sullivan added: "Happy to be back in snooker. See you all soon xx"
After beating Ali Carter 18-11 in the World Championship final on May 7, O'Sullivan said: "I work as hard as anyone in snooker and I just want to be treated fairly.
"That's up to the governing body to treat players right."
The growth of the professional tour since Barry Hearn stepped in as World Snooker chairman in December 2009 has been a factor in O'Sullivan's growing frustration.
He said in May there were "certain pressures that I can't do with anymore", and the calendar becoming crammed with tournaments carrying world ranking points had meant there was added pressure to appear everywhere.
Hearn's contention is that snooker has never offered so many opportunities for players to earn.
Last season O'Sullivan, the sport's guaranteed crowd-puller, was close to falling out of the top 16 and having to go through qualifying to reach round one of the World Championship.
And while he eventually made the cut and his world title lifted him to ninth, by remaining inactive since then the Essex-based cueman has slipped to 15th place.
His return to top-level snooker is set to come at the new International Championship in China, which runs from October 28 to November 4, offering a top prize of £125,000 and the same number of ranking points as the UK Championship.
Looking ahead to his late-October return, O'Sullivan said: "I have taken the six-month sabbatical I said I would and I am now looking forward to competing over the rest of this season. I am happy to be back in snooker and seeing all my fans soon!"
Hearn added: "I met with Ronnie and following our discussion he has signed the WSL (World Snooker Limited) players' contract.
"We are obviously very pleased that Ronnie has decided to come back to snooker, the sport he loves. This is great news for his many fans around the world. We welcome him back with open arms."