O'Sullivan has won the tournament four times - a competition record - but pulled out of the event, which began this week, on Tuesday.
He beat Judd Trump (34) 10-6 in last year's final.
"Mentally I feel a bit drained and stressed," wrote O'Sullivan in a post on X, formerly known as Twitter.
"I want to look after my mental health and my body. I'm sorry to all the fans but I'll be back stronger."
Ding Junhui (36) will replace O'Sullivan for Wednesday's match with Zhang Anda (31) in Bolton.
The Englishman has pulled out of high-profile tournaments on numerous occasions in the past, most recently at the Northern Ireland Open in Belfast in October.
He has also spoken publicly of his mental health issues in the past, as well as his much-publicised battles with alcohol and drugs.