El Patroness will join Vow And Declare as O'Brien's runners in the $500,000 Listed event at Flemington.
The Andrew Ramsden Stakes carries a Melbourne Cup ballot exemption and will draw a strong field, with White Marlin and Hezashocka among the opposition, which O'Brien considers a good test for El Patroness in particular.
"White Marlin is the Melbourne Cup favourite and Hezashocka is running, so it's going to be a good edition of the local chances for the spring and we'll see where we sit after the race."
El Patroness heads into the Ramsden Stakes off a strong win in the $250,000 VOBIS Gold Heath at Sandown on April 15.
That 2100m event was her first win since the Group 1 ATC Oaks (2400m) 12 months earlier and while it will be the longest race she has contested, O'Brien is confident in the Shamus Award four-year-old's staying credentials.
"We've only seen her once out at a mile-and-a-half, which was the Oaks, and she was very impressive so we're keen to have another look at her over a staying trip," O'Brien said.
"She's had a good grounding for it, she seems to have done well since she won well at Sandown and I'm looking forward to running her. It will answer a lot of questions for us as to what her scope is going forward."
El Patroness and Vow And Declare tuned up for the Ramsden with 1000m jumpout at Flemington last Friday, when they were ridden out for an extra 400m after the finish point.
O'Brien saw enough to suggest Vow And Declare can return his form from earlier in the campaign, which included a fourth placing in the Group 1 Tancred Stakes (2400m), after disappointing when sixth in the Listed Mornington Cup (2400m) last time out.
"It's not a track that every horse gets around and he raced a few lengths off his previous form and I think as much as anything you just put that down to not being comfortable around Mornington," O'Brien said of the 2019 Melbourne Cup winner.
"He gets back to Flemington next Saturday and that's a track that he's always had a great affinity with."
The road to the Lexus Melbourne Cup continues on Saturday 13 May when The Listed Lexus Andrew Ramsden provides another opportunity to win a Golden Ticket into Australia's most famous thoroughbred race. Tickets selling fast, don't miss out!