A testing gallop at Rosehill on Tuesday morning will determine whether Oakleigh Plate winner Mrs Onassis returns to the scene of her greatest triumph four days later.

The mare won the Group One sprint at Caulfield in February and is entered for the Sir John Monash Stakes over the 1100m Oakleigh Plate course on Saturday.

Trainer Gerald Ryan said her winter campaign could be her last with owner Gerry Harvey still undecided if Mrs Onassis would be sent to the breeding barn this year.

"She had a jump-out last week and if I'm happy with her after she works in the morning she will go to Melbourne and Brenton Avdulla will ride," Ryan said.

"I'm not sure if she will be racing into the spring.

"It (stud) has been touched on but there is still no decision."

The Listed Sir John Monash attracted 18 entries on Monday including dual Group One winner Atomic Force who will work at Newcastle on Tuesday before a decision is made on his return to racing after a three-month break.

Wet track specialist Second Effort, winner of the past two runnings of the Listed Wangoom Handicap at Warrnambool has the equal highest rating of 105 alongside Mrs Onassis.

Skytrain is next on 104 with Darley preferring the Caulfield sprint to Wednesday's Ramornie Handicap at Grafton.

Weighing heavily in Mrs Onassis' favour is her record of seven wins over 1100 metres, a distance which has so far been her optimum.

Also among the entries is three-year-old Happy Galaxy from the Hawkes stable.

The colt climbed up the ranks in the summer with four straight wins, culminating in the Group Two Expressway Stakes in February in which he beat Skytrain.