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The current world number seven lost the opening frame but quickly levelled with a break of 73 in the second.
Australian left-hander Robertson then had runs of 53 and a frame-winning 80 to take the third, only for O'Sullivan to delight the bulk of a sold-out crowd by levelling after a tight fourth in which both players had breaks of 67.
'The Rocket' started superbly after the interval with the highest break of the tournament, a 143 clearance, only for Robertson - who had beaten world champion Mark Selby and home favourite Marco Fu to reach the final - to level it up again at 3-3.
He edged a closely-contested seventh frame 55-54 before beaks of 68 and 82 saw him clinch his first victory of the new season and claim a cheque for £100,000. It’s the third invitational title of his career following victories at the 2012 Masters and 2015 Champion of Champions.
Robertson told www.worldsnooker.com: "This event had the best eight players in the world. The draw I had was incredible. Mark Selby, Marco Fu and Ronnie O’Sullivan.
"It doesn’t get any harder than that. To do it in front of three thousand people was amazing. That was the best playing experience of my career.
"Last season was really bad by my standards and I think I needed that to happen to know what to do going forward. This was the first big tournament of the campaign and my preparation was fantastic.
"I've worked really hard and I just have to do this for the rest of my career. I need to play this aggressive as well. I didn't let Ronnie dictate the pace of the match.
"I'm not going to target certain events. I just need to practise from start to finish. Mark Selby has really inspired me. Seeing how hard he works. Usually I don’t do much practising during events. But I have done a little bit extra during this week and it showed."
O’Sullivan admitted he needs to make a change with his cue.
He said: "If at the start of the week you told me I would be in the final I would have taken it. I don’t make excuses but I will be getting rid of my cue after this week, it’s finished. There were certain shots I couldn’t play as it has lost it’s feel. I just have to take the positives out of this week."
Hong Kong Masters Results
First Round (Best of 9 frames)
Barry Hawkins 3-5 Marco Fu
Mark Selby 3-5 Neil Robertson
Judd Trump 5-3 Shaun Murphy
John Higgins 4-5 Ronnie O'Sullivan
Semi-Finals (Best of 11 frames)
Neil Robertson 6-4 Marco Fu
Judd Trump 5-6 Ronnie O'Sullivan
Final (Best of 11 frames)
Neil Robertson 6-3 Ronnie O'Sullivan
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