Ridden by Damian Lane, Grinzinger Belle ($12) scored by 1-¼ lengths from French Endeavour ($18) with the $4.60 favourite Mollynickers a nose away third.
Grinzinger Belle was tried over 2000m during the spring when midfield in the Ethereal Classic at Caulfield before backing-up the following week to finish second at Moonee Valley over 1600m.
After Saturday's win, O'Brien said he was likely to sidestep the Group 1 Australian Guineas (1600m) at Flemington on March 2 with the Group 2 Kewney Stakes (1600m) at the track the following week a more likely option.
"I think she is 1400 (metres) to a mile and a strong on-speed filly at that distance," O'Brien said.
"I don't think she is a genuine stayer.
"She is more likely to stay amongst the fillies in three weeks' time in the Kewney and then we're back to Moonee Valley for another fillies' races.
"There's two good Group races for her in Melbourne."
O'Brien said Grinzinger Belle, a daughter of Shamexpress that ran second in the Lightning Stakes on the corresponding day 10 years ago, had spelled well over summer.
He said the filly also has the strength to carry her frame, although he was a little concerned early in the race with the sectionals the filly running.
"We had her forward enough to run well but when I was watching those sectionals and they were running 10.9, 11 and a tick, 11 and a tick, you're always never sure how much petrol was going to be left in the end," O'Brien said.
"But fair play to her, she was solid right down the running, and it was a very good win."
