The first frame was tight and tetchy and did not portend what was to follow, with the Rocket triumphing 89-16 in over half an hour.
- Murphy upsets Trump to reach last four, Bingtao beats Higgins
- Watch the Welsh Open live on Eurosport and the Eurosport Player
- ''There was just nothing there' - Robertson reveals mental exhaustion behind defeat
- 'I had a plan' - Ronnie reveals his strategy for smashing Selby
However, the second frame was over in a flash, with O'Sullivan, a four-tie champion at this event playing some stunning shots and recording a break of 142 in a matter of minutes.
'What a break that was!' - O'Sullivan hits brilliant 142 on way to Selby victory
In the third, he fell agonisingly short of back-to-back centuries but still won 95-0 to take complete control of the match and another break of 85 put him one frame away from victory.
In the fifth, Selby had a run on the table for the first time and recorded a break of 37. However, a string of errors from both players led to a lengthy battle for the red, which Selby eventually won, averting the whitewash.
O'Sullivan: You have to play the game on your terms
With Selby in a decent position in the sixth frame, an unfortunate ricochet potted the cue ball, leaving O'Sullivan to finally seal a dominant win.
In the semi-final he will face Kyren Wilson, who demolished Neil Robertson in an even more one-sided affair 5-0.
Robertson admitted after his first-round win that he was exhausted and he looked a tired player unable to make a break of note as Wilson, in contrast, looked in control of his game.
The first frame was a scrappy affair that Wilson won with classy positional play netting the last three balls placed in awkward positions and then a break of 79 saw him double his advantage.
Robertson: There was just nothing there, mentally I couldn't get up for it
Both players had opportunities in the third, but Wilson was more convincing and then an audacious double of the yellow effectively put him 4-0 up at the interval.
The last frame followed a similar path with Wilson, although not scoring heavily, doing enough to put the frame beyond his under-par opponent.
Shaun Murphy faces Yan Bingtao in the other semi-final after defeating Judd Trump.
'There was just nothing there'
“It was the performance I was expecting to happen earlier in the week,” Robertson told Eurosport.
Last night actually gave me a lot of hope but I woke up today and when I got out there, there was just nothing there. Physically absolutely fine, just mentally I couldn’t get up for the match.
“I tried to make myself try, but I don’t know, there was just nothing there. The best way to play is when you are not really under pressure but you just have a little bit of nerves ticking away… I had nothing at all. I was kind of like the same pressure you have in practice.
'He must be exhausted'
Robertson spoke earlier in the competition about how tired he was following a bruising schedule and Eurosport expert Jimmy White said that was evident in his defeat.
Take nothing away from Kyren Wilson – he played solid, his potting was as good as ever and his cue ball control was good as well. I’m not saying [Robertson] would have beaten Kyren Wilson today, but he didn’t really compete. He has got to be exhausted.
And it was a point echoed by Reanne Evans...
You can’t blame him for coming here, defending champion, maybe he’d have pulled out if he wasn’t defending. Trust me he gave it 100% but you could just tell in the last few frames his concentration wasn’t there, he was missing balls that you never expect him to. Fair play to Kyren, he dug in and got the win.