"The way it fell it looked like he was going to be a horse that would be really suited in the Melbourne Cup.
"The Melbourne Cup came and he was too low (in the weights) - he couldn't get in - so we had to run him (in the Herbert Power Stakes) and he had to win to get in and so we ran him and we saw what happened.
"We didn't run him again because we felt that he was in on the perfect mark.
"He loves fast ground and another reason he might not have got that ever there (Europe). The better the ground, the better his chance.
"Everything seems to be going well so it will be very interesting to see with him."
O'Brien has saddled nine runners in the Melbourne Cup.
He came closest last year when Johannes Vermeer was run down late by his son Joseph's horse Rekindling. "It is a very difficult race to win," O'Brien said. "You have to have a horse in on a nice mark. You have to have class and you have to stay.
"Yucatan looks to be on a very good mark. Rostropovich looks to be in on a very good mark. We're happy with Cliffs Of Moher but he's a very well exposed horse."
Rostropovich, with 51kg, is half a kilogram under the weight Rekindling carried in the race when a northern hemisphere three-year-old.
"He was a horse that we always thought would stay forward in a mile and a half," O'Brien said.
"He seems to be on a lovely weight. When he went down we felt we needed to give him a run to sharpen him up and that's why we ran him at Moonee Valley (Cox Plate) over a mile and a quarter.
"Ryan (Moore), who rode him there, was very happy with him and felt that obviously going further would suit him.
"Wayne (Lordan) is going down to ride him. He is just perfect in on the bottom (weight)." O'Brien is wary that The Cliffsofmoher might find his handicap of 56.5kg a task.
"He obviously is a very classy horse," he said.
"He has plenty of weight but we were very happy with his two runs and we're looking forward to seeing how we go.
"The Melbourne Cup is a tough, difficult race and you are never sure what horse is going to be suited or what is going to work but we always thought that he's a horse that could run a big race in it.
"Ryan will probably take his time on him and we'll see what will happen."
• Kiwi jockey James McDonald has a golden opportunity to win Australasia's greatest race today when he partners Yucatan in the Melbourne Cup.
Victory in the two-mile feature would be extra special for the boy from Cambridge, who took all before him as an apprentice jockey in New Zealand before shooting to prominence as a star of the international jockey ranks.
McDonald will be having his fifth ride in the Melbourne Cup today and hopes he can improve on his placed efforts with Fiorente (second, 2012) and Hartnell (third, 2016).
"It's really satisfying to be on a live chance in the race and he's with the best possible team," McDonald said.
THE BIG FOUR
• YUCATAN
• AVILIUS
• ROSTROPOVICH
• MAGIC CIRCLE
MELBOURNE CUP 3200m A$7,660,000
1 X1111 Best Solution (6) 57.5 Pat Cosgrave $13.00 $4.20
2 33X43 The Cliffsofmoher (9) 56.5 Ryan Moore $16.00 $4.80
3 0X11X Magic Circle (17) 56 Corey Brown $9.00 $3.20
4 250X0 Chestnut Coat (4) 55.5 Yuga Kawada $61.00 $17
5 X4341 Muntahaa (13) 55.5 Jim Crowley $11.00 $3.50
6 11320 Sound Check (16) 55.5 Jordan Childs $31.00 $8.50
7 X7694 Who Shot Thebarman (18) 55.5 Ben Melham $41.00 $10.00
8 02410 Ace High (22) 55 Tye Angland $61.00 $17.00
9 X2112 Marmelo (10) 55 Hugh Bowman $14.00 $4.50
10 11114 Avilius (11) 54.5 Glyn Schofield $13.00 $4.20
11 60131 Yucatan (23) 54.5 James McDonald $6.00 $2.40
12 X5266 Auvray (1) 54 Tommy Berry $101.00 $31.00
13 65513 Finche (15) 54 Zac Purton $26.00 $7.25
14 08030 Red Cardinal (5) 54 Damien Oliver $35.00 $9.00
15 X4059 Vengeur Masque (2) 54 Patrick Moloney $61.00 $16.00
16 79401 Ventura Storm (7) 54 Mark Zahra $35.00 $9.00
17 32X31 A Prince Of Arran (20) 53 Michael Walker $17.00 $5.00
18 05770 Nakeeta (3) 53 Regan Bayliss $71.00 $19.00
19 01473 Sir Charles Road (14) 53 Dwayne Dunn $61.00 $7.00
20 X0080 Zacada (24) 53 Damian Lane $81.00 $21.00
21 0X341 Runaway (12) 52 Stephen Baster $41.00 $9.50
22 36327 Youngstar (8) 51.5 Craig Williams $16.00 $4.80
23 24112 Cross Counter (19) 51 Kerrin McEvoy $9.00 $3.20
24 22515 Rostropovich (21) 51 Wayne Lordan $21.00 $6.25