The new world number one Selby always looked the likelier winner of the tournament once he managed to move 12-11 clear after a turgid afternoon session that saw just six frames completed in over three hours.
But while O'Sullivan was forced to sit and suffer against an opponent he once nicknamed 'The Torturter', Selby seemed to prosper as his game improved the longer the final progressed. In the end, O'Sullivan could have few complaints having lost 11 of the closing 15 frames..
Selby is not as eye-catching as O'Sullivan, but he is a worthy world champion after the latest 17-day event in England.
He played some magnificent matchplay snooker in the evening session with O'Sullivan - bidding for a sixth world title after winning his last two in 2012 and 2013 - finally found wanting with too many safety errors and missed opportunities among the balls costing him his prospects of a third straight title as Selby's tactical nous became the dominant force.
The man from Leicester lost the 2007 final to John Higgins, but all good things come to those who wait. His reward is the famous old pot and a cheque for £300,000 as an emotional Selby dedicated his success to his late father, who he lost at the age of 16.
O'Sullivan must wait for next year in his bid to equal the hauls of Steve Davis and Ray Reardon with their six world titles. Stephen Hendry's record of seven remain tantalisingly on the horizon, but he must wait for another crack at regaining the crown.
He should take heart from his performance in suffering his first defeat at the Crucible since losing to Higgins in the 2011 quarter-final.
But the game's greatest player will look back on holding a 10-5 lead on Sunday night, and realise he missed an opportunity to lead by six or seven frames before they resumed this afternoon rather than just the 10-7 he was forced to settle on.
O'Sullivan levelled the match at 12-12 from trailing 12-11 with a 100 break only for Selby to dominate the next three frames despite only making 56 in the 27th frame.
Runs of 30 and 49 saw O'Sullivan close to 15-13 behind, and that became 15-14 with an immaculate clearance of 37 in the 29th frame.
Despite that inspirational moment, there was a feeling O'Sullivan was kicking against the wind as runs of 127 - Selby's first century of the final - and an 87 left 'The Jester' one frame short of the winning post.
O'Sullivan broke down holding a 56-0 lead in the 32nd frame as Selby returned to the table with a 35 clearance giving him the title to spark some hectic celebrations.
Mark Selby leads Ronnie O'Sullivan 12-11 in a World Championship snooker final that has turned into survival of the fittest at Sheffield's Crucible Theatre.
Whether or not Selby's matchplay tactics are to your liking, it was quite disgraceful that only six frames of snooker were played from the back of 2pm (BST) as both players were hauled off three-and-a-half hours later with two frames still to be played in the afternoon session.
O'Sullivan - whose nickname for Selby of 'The Torturer' seems curiously apt - lost his 10-7 lead and seemed close to losing his head as he missed a straight pink in the 23rd frame that would have given him a 12-11 advantage in his quest to equal Steve Davis and Ray Reardon's modern day haul of six titles.
In hindsight, perhaps the miss was understandable with Selby fraternising with the level of tedium that watching Canadian Cliff Thorburn used to specialise in back in the 1980s.
The player from Leicester - who ran in 55, 52 and 72 to draw level at 10-10 - pumped his fist as if he had won the tournament when he took out pink and black to punish O'Sullivan's astonishing miss on pink, but there are not many neutrals watching who will be sharing in his glee.
It was not good to watch back in Thorburn's era, and it is one of the reasons why snooker has lost its popularity among some parts of the public with both players knocked out of their rhythm and frames becoming tactical frames of attrition that are hardly a good advert for the showpiece occasion.
If such levels of slow play continue in the evening, the final could be set for a late, late finish with another 12 frames possible in the race - or crawl to reach - the winning mark of 18 frames.
Most neutrals will hope O'Sullivan - whose highest break of 50 in the 22nd frame helped him snatch a frame he could easily have lost - can click back into gear with his trademark fluency going missing at the key moments at the Crucible.
Ronnie O'Sullivan moved closer to a sixth world title as he built up a 10-7 overnight lead over Mark Selby in their World Championship final at Sheffield's Crucible Theatre.
Despite running in two centuries in the final that helped him to a 10-5 advantage, Selby kept alive his hopes of an unlikely recovery by picking up the final two frames of the session to trail by three frames when he could have been further adrift.
The third session that begins at 2pm on Monday is likely to be crucial to 2007 finalist Selby's hopes of remaining viable in the final with O'Sullivan winning the first two sessions 5-3 and 5-4 to illustrate why he looks likely to lift his sixth world title on Monday night.
It has been an engrossing enough encounter, but slightly underwhelming with Selby's highest break being only 62. He should probably be further behind, but will take heart from somehow managing to stay with O'Sullivan, who has been businesslike without ever hitting his peak levels so far.
Resuming 5-3 ahead, O'Sullivan won the first three frames of the evening session aided by runs of 52 and 63 to lead 8-3.
A 55 saw Selby claw one back to trail 8-4 before he somehow scrambled over the line in the 13th frame after O'Sullivan had got the snookers he needed to recover in the frame.
He was so unfortunate to pot a red and see the green disappear down a baulk bag with Selby returning to mop up in closing to 8-5 behind.
O'Sullivan dominated the next two frames with breaks of 131 and 87, but 58 and 62 from Selby in the final two frames of the night will at least give him fresh hope heading into the third session.
O'Sullivan is hoping to equal the modern day total of Steve Davis and Ray Reardon, who both have six world titles to their name. Stephen Hendry's seven remains the target O'Sullivan is hoping to move closer to this year.
Mark Selby missed a key black in the eighth frame as Ronnie O'Sullivan escaped from the first session of the World Championship final holding a 5-3 lead at Sheffield's Crucible Theatre.
Defending five-times' champion O'Sullivan is aiming to become the first man to win three straight gongs in Sheffield since Stephen Hendry enjoyed his purple patch between 1992 and 1996 in making his way to five on the bounce amid his record collection of seven.
Chigwell's O'Sullivan has history and Hendry's total firmly in his sights these days. He snagged the first three frames of his sixth Crucible final with consummate ease, but required an awful miss by Selby in the death throes of the first session to ensure he eked out a lead he certainly earned before the second session begins at 7pm.
Having fluked the final red in making a mess of an intended safety, Selby failed to cash in on his good fortune as he missed a simple black. Having watched Selby land perfectly on yellow from black, O'Sullivan swept home the colours for a two-frame advantage in the knowledge that he had struggled to gain his trademark momentum earlier in the frame.
Upon reflection, Selby cannot be too sore about being only two frames behind, but he will know the unlikely prospect of 4-4 was viable despite astonishingly making a highest break of only 38 in the opening eight frames.
That statistic will have to improve dramatically if Selby has any chance of living with O'Sullivan over the next three sessions.
The world champion will also be disappointed he did not win the session by a heavier margin having squandered decent chances among the balls in the fourth and fifth frames with Selby struggling to score heavily.
He was 3-0 up inside an hour with breaks of 69 and 102 in frames one and three with Selby looking slightly overawed by the occasion in his first final appearance at the tournament since he lost to John Higgins in 2007.
A lovely break of 63 from a long red handed O'Sullivan the sixth frame, but the fourth, fifth and seventh frames were all dominated by Selby's stickability. His penchant for grinding matchplay snooker seemed to knock O'Sullivan off his stride for a few frames with some loose safety shots particularly heartening for the Selby cause.
Nine more frames are scheduled from 7pm with the first man to 18 frames claiming the trophy and a £300,000 cheque on Monday evening.
Sunday, 4 May
Final (best of 35 frames)
Ronnie O'Sullivan (Eng) 5-3 Mark Selby (Eng)
Ronnie O'Sullivan (Eng) 10-7 Mark Selby (Eng)
Monday, 5 May
Final (best of 35 frames)
Ronnie O'Sullivan (Eng) 11-12 Mark Selby (Eng)
Ronnie O'Sullivan (Eng) 14-18 Mark Selby (Eng)
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