The stayer has won just two races since his 2019 Melbourne Cup triumph, the Group 2 Sandown Classic (2400m) at Caulfield in November 2022 and when defeating stablemate Young Werther in the Group 3 Tatt's Cup (2400m) at Eagle Farm in June.
In his solitary run since that Eagle Farm victory, Vow And Declare finished fifth at Caulfield over 2000m on August 31.
"I thought he was good at Caulfield the other day and he's obviously better suited again up to the 2500 metres back at Flemington," O'Brien said.
"Hopefully he's going to run really well again.
"He's nine, but he's only had 40 odd starts and it's not like he's not enjoying it. He's sound and healthy and well and while he's like that, he's happiest while he's with us at the stable.
"It's all he knows. He's been with as for eight years. He came to us as a yearling."
O'Brien said Vow And Declare was a fan favourite.
"The public has been great with him," O'Brien said.
"They have always loved watching him go to the races and it's been good that he's been competitive and always run well.
"It would be lovely to win another race with him this spring in Melbourne."
O'Brien is hoping to turn around the fortunes of Sea What I See in the Ronald McDonald House Charities Trophy (1700m).
See What I See strung together six wins on the trot during her last campaign but resumed at Moonee Valley last month finishing last over 1600m.
"She was disappointing first-up, but we can't fault her," O'Brien said.
"We're going to have another look at her on Saturday, and if she doesn't run well, she can go for another break."
