The controversial O'Sullivan, widely regarded as the most naturally-gifted snooker player of all time, won his fourth world title last May when he defeated Ali Carter in the final in Sheffield.
O'Sullivan also bested Carter in 2008 after triumphs in 2004 and 2001, but has sat out the majority of the 2012/13 season after growing disillusioned with the profession and what he regarded as an overly-demanding schedule, whilst also battling depression.
The 37-year-old had until February 28 to enter, and as defending champion will walk straight in as the top seed despite falling to 20th in the provisional world rankings due to inactivity.
"I've had a nice year out, I've enjoyed it, had a lot of fun, I needed the rest but I just thought it was time to get back to doing what I've done for a lot of my life," O'Sullivan said at the press conference in London.
"Three or four months ago I was sitting there thinking I'd take being beaten 10-0 at Sheffield just to be back playing rather than just sitting there, not really having much going on in my life other than getting up and going out for lunches and dinners and chilling out.
"I got a bit bored of that to be honest."
The 2013 edition of the sport's marquee tournament runs from April 20 through to May 6, and O'Sullivan admits he is unlikely to be odds-on to retain after playing just a handful of exhibitions over the last several months.
"I'll be very match rusty...it's going to be a tough call to go in there with no match practice behind me...it's going to be a massive challenge but I just see this as the start of the bigger picture really," he said.
"I'm used to being written off and coming back. I love the challenge, I love the game, I know how good I am, I know what I'm capable of and I hope some people do write me off as that will make me even stronger."
The Sun reported last month that 'The Rocket' had been working as a voluntary labourer on a farm in Essex, which O'Sullivan admitted was purely to keep himself busy without snooker.
"I've been cleaning out stables and pig sties, taking down fences, putting rubbish into barrels and clearing mud," he told the newspaper.
"It has been mainly manual work, also getting big bales of hay in to feed the animals. I was getting so bored I had to do something, and needed a goal to get out of bed in the morning.
"There are personal things I need to sort out before I can even think of a return. I have until February to make that decision about the Crucible, anyway, but I am still playing every couple of weeks at my mum's house."