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Australian Robertson, 34, had won the first two frames of the best-of-17 final and extended his lead to 5-2 during the first session in Cardiff.
But O'Sullivan, 40, won the next seven frames, sealing victory with a break of 141 to move level with John Higgins on four Welsh Open titles.
"I feel shattered, I need a month of just relaxing," said the Englishman.
O'Sullivan caused controversy earlier in the week when he turned down the opportunity of a 147 break, making 146 instead.
He also spoke about his boredom at tournaments.
But he said he had enjoyed a "fantastic week" and would now be resting before the World Championships in April.
"That is the one that everyone wants," he told BBC Sport.
"I am not eligible for any other tournaments as I did not qualify, so my cue will be chilling out for a month or so."
The 2007 champion Robertson, who was described by O'Sullivan as "the best player in the world", pinpointed the ninth frame as the key moment.
"Once he got his tail up and the momentum it was hard to wrestle it back from him," said the world number three.
"Everyone knows he is a terrific front runner and he pulled away and finished the match off in style. I made mistakes and that is what happens when you are put under pressure."
36 - Stephen Hendry
28 - Ronnie O'Sullivan, John Higgins, Steve Davis
18 - Mark Williams
Ronnie O'Sullivan won seven frames in a row to beat Neil Robertson 9-5 and win his fourth Welsh Open title.