Defending Welsh Open champion O'Sullivan is also out after he saw Mark Davis hit back from trailing 3-0 to complete 4-3 win at the Motorpoint Arena.
The 44-year-old looked set to be whitewashed when he trailed O'Sullivan 3-0 but a nerveless break of 81 helped him drag the match back level before claiming victory in the decider.
Fellow veterans Mark King and Anthony Hamilton have already clinched their first ranking titles this season and Davis said he now sees no reason why he cannot follow their examples.
Davis told Eurosport: "I watched Mark and Anthony and they have both been around a long time.
They have managed to hold themselves together and do the business. You've just got to be persistent and keep going, and go out and do it for yourself.
On a day of shocks Robertson was beaten 4-3 by Lee Walker while Shaun Murphy, Marco Fu and Liang Wenbo also lost.
World champion Mark Selby avoided the same fate but only after twice having to come from behind to overcome Mike Dunn 4-2.
World number seven Robertson lost a close first frame before making immaculate breaks of 133, 143 - the highest of the event so far - and 55 to move 3-1 clear.
But he was forced to sit and sutffer as Walker won the next three frames - the fifth by the aid of a fluked final black - before compiling a brilliant 130 of his own in the decider to complete one of the biggest wins of his career.
In a day of shocks, Shaun Murphy also departed the tournament after suffering a 4-2 defeat to Josh Boileau.
Read the full article on eurosport.co.uk: O'Sullivan and Robertson lose in Welsh Open second round
- Sports & Recreation
- Mark Davis
- Shaun Murphy
- Lee Walker
- Mark King
- Anthony Hamilton