O'Sullivan is also banking on a return to Flemington suiting the filly.
"She was very tough at the end of 2000 metres in the Ethereal, so I've got to be guided by that," O'Sullivan said.
"I think she will be better suited on the bigger, flatter track at Flemington, and I think she looked better in the Edward Manifold than what she did at Caulfield, so you have to back yourself.
"She's a very tough filly and we've done the work for her to run out the trip, and I think we have done that, so the proof will be on Thursday."
Too Darn Discreet holds a nomination in the Group 1 Thousand Guineas (1600m) at Caulfield on Saturday week, but O'Sullivan said the filly would be in the paddock by the time that race is run.
"I put her in as a back-up, just in case there was a simple thing like she pulls a shoe off and gets a foot abscess, or something goes wrong, and you haven't put yourself in there," O'Sullivan said.
"You're then in the paddock instead of going to a race that would have been an option, but it was purely as a back-up.
"She'll be in the paddock after the weekend.
"We'll give her a few days to let down and as she's owned in South Australia we'll prepare her for Adelaide and the Australasian Oaks."