It's all or nothing for on-pace mare Oasis Bloom who needs to win Saturday's Herbert Power Stakes to qualify for a start in the Caulfield Cup.

Peter Morgan, who trains the five-year-old in partnership with Craig Widdison, says they have no back-up plan if she fails to secure a Cup berth.

"If we don't win...well, I haven't thought beyond Saturday. There is no Plan B so I hope we can get there," Morgan said.

At her last start in the Group Three JRA Cup (2040m) at Moonee Valley, Oasis Bloom took up her usual position close to the lead but she struggled to find clear galloping room in the straight and finished fifth.

"She's a big horse and shouldn't have been near the fence - she likes to race with a bit of room," Morgan said.

"But if she gets a mile-and-a-half she'll be hard to beat, she's improved since that race."

Oasis Dream has never been tested beyond 2040m and she will be up against horses that have high-class formlines over the Herbert Power journey of 2400m and beyond.

They include Irish import Simenon, who came second to the Queen's horse Estimate in the Group One Ascot Gold Cup and Sea Moon, who was first past the post in the Bart Cummings last Saturday only to lose the race on protest.

"That's the only doubt, whether she'll get the mile-and-a-half, but she's a lot fitter and will be in a lot better order than she was for her last run," Morgan said.

Oasis Bloom spent almost 10 months on the sidelines after suffering a bone chip in the Group One Myer Classic last November and Saturday's race will be her fifth this campaign.