Young Werther will be racing for the 11th time in Saturday's Group 1 handicap at Caulfield, but O'Brien said the four-year-old has been conditioned for the race.
A maiden winner on debut over 1500m, the only time Young Werther has been successful, the now four-year-old has been geared for a Caulfield and Melbourne Cup campaign from his early days.
Immaturity held Young Werther to last spring, but O'Brien saw enough in the son of Tavistock to aim at the Group 1 Victoria Derby (2500m) at Flemington at his third start.
As an autumn three-year-old, Young Werther had five starts, culminating with a narrow second to Explosive Jack, an opponent on Saturday, in the Group 1 Australian Derby at Randwick.
"Those runs in the autumn seasoned and conditioned him up," O'Brien said.
"He was also a horse that was a bit immature, and we were always conscious of not wanting to do too much as a three-year-old, albeit we did run him in both Derby's, but they were moderately run and weren't tough events."
O'Brien said Young Werther had made progression this campaign.
After a first-up fifth in a 1700m handicap at Flemington on September 11, Young Werther almost lowered the colours of Incentivise in the Group 1 Turnbull Stakes (2000m) at Flemington on October 2.
"He's run well in the Turnbull which is a good race to run well in as far as heading to the Cups and he's improved out of it," O'Brien said.
"He's conditioned enough now as a spring four-year-old to go to the Caulfield Cup, one of the toughest mile-and-a-half races in the world.
"It will be a proper test on Saturday, and I think that's what he's looking for now."
After drawing barrier eight on Wednesday, Young Werther is now the outright third favourite at $10 with TAB after his three main opponents in the market – Incentivise ($2.50), Delphi ($8) and Nonconformist ($12) – all drew wide.