The two English players shared the first four frames before O'Sullivan hit a 124 clearance to edge ahead in the semi-final in Cardiff.
O'Sullivan, a three-time champion, then made breaks of 94 and 88 to lead 5-2 before winning with a 78.
Australia's Robertson beat Northern Ireland's Mark Allen 6-4 to reach his second final, to be played on Sunday.
Robertson, the 2007 champion, had trailed 2-0 in the best-of-11 frame match, but won the crucial third frame after a foul by Allen, of Northern Ireland.
The world number three then won five consecutive frames to lead 5-2 and withstood a comeback by Allen to reach the final.
Speaking after the semi-final, O'Sullivan, 40, compared Robertson to cinematic cyborg law enforcer RoboCop.
"I have watched him through the UK Championships and he never showed emotion through the whole tournament, " O'Sullivan told BBC Sport, ahead of the best-of-17 final.
"It shows what an unbelievable professional he is. He is Robocop, he is like the new Steve Davis or Stephen Hendry."
Robertson, 34, believes O'Sullivan is the favourite for the title.
"I don't mind flying under the radar," he added. "It is nice, I can be the underdog."

The 24-year-old, whose current contract ends at the end of 2016, was offered a four-year extension by the Scottish Premiership leaders in January.
Forrest came through the youth ranks with the reigning champions, for whom he has made 144 appearances.
However, his outings have been restricted by a series of injuries.
Forrest, who has 13 Scotland caps, was missing from Celtic's squad for Saturday's 3-0 victory over Inverness Caledonian Thistle.
Celtic signed Manchester City winger Patrick Roberts on an 18-month loan deal from Manchester City in January and last week secured a summer move for 17-year-old midfielder Kristoffer Ajer from Norwegian club Start.
Deila admitted on Saturday that Celtic had too many midfielders on their books and that he would have to slim his squad at the end of this season.
Ronnie O'Sullivan beat Joe Perry 6-3 to reach the Welsh Open final where he will play Neil Robertson.
[NEXT_CONCEPT]
Norwich City and Queens Park Rangers are among clubs showing an interest in James Forrest after the Scotland winger rejected a new Celtic contract.