O'Mara (right) will miss out on the big race
154th University Boat Race
Oxford v Cambridge
Date: Saturday, 29 March
Start: 1715 GMT
Venue: River Thames - Putney to Mortlake
BBC Coverage: Updates on BBC Radio 5 Live
Cambridge have been dealt a massive blow just three days before the Boat Race after their stroke Shane O'Mara was ruled out on medical grounds.
Ryan Monaghan, 23, will step up from the reserve crew into the most important seat in the Cambridge Boat.
Lighter and less experienced than Oxford, Cambridge were already underdogs ahead of the 154th Boat Race.
"On the day we will put out the best eight we have and we believe we can win," said coach Duncan Holland.
Stroke is a vital position in any crew, responsible for setting rhythm and rating.
Monaghan, from New York state, took the seat in training on Wednesday evening, although the order of the crew could still be changed.
O'Mara appeared at Tuesday's weigh-in and took part in training later in the day but became ill overnight and was ruled out after visiting a doctor on Wednesday.
Holland refused to go into the details of American O'Mara's illness and requested privacy for the 26-year-old.
"The medical advice is that he won't be ready to race on Saturday so with a great deal of sadness we've agreed that Shane is not racing," he added.
Coxes Dowbiggin (left) and Brodie pose with the Boat Race trophy
Cambridge were hoping that their technique would help to overcome a weight disadvantage of 12lb per man to Oxford on Saturday.
The holders weighed in on Tuesday at an average of 14st 9lb but a forecast of 22mph winds in London could turn the Boat Race into a dogfight.
Holland said: "Historically the heavier crew tend to win but a lot could depend on how the crews react to the conditions."
Oxford coach Sean Bowden added: "I don't mind being the heavier crew."
Bowden's men weighed in at 15st 6lb on average, with Australian Toby Medaris the heaviest at 16st 2lb.
"I am happy to go in with strong, athletic, tall people but there's more to it than just size," said Bowden.
Traditionally a heavy crew goes well in a head wind, less well in a tailwind
Cambridge coach Duncan Holland
"We didn't pick the crew on size - we did it on ability and performance."
New Zealander Holland has unpleasant memories of his first race in charge, in 2006, when Oxford took advantage of rough water in the middle of the 4.25-mile course on the River Thames to break clear.
"The Championship course is a fascinating piece of water," Holland explained.
"Because of the shape of the river and the tide coursing through it, conditions vary greatly from one part of the river to another so crews need to adapt on the run.
"Traditionally a heavy crew goes well in a head wind, less well in a tailwind so each crew will find conditions to suit."
Both crews will be boosted by the return of their coxes from last year's race.
Rebecca Dowbiggin steered Cambridge to victory by one-and-a-quarter lengths last year and is aiming to become the first female Light Blue cox to win the race twice.
Nick Brodie returns as president of Oxford, aiming for revenge.
With Olympic demands cutting current internationals from both squads, Cambridge have the only oarsman to have taken part in the race before - Australian Tom Edwards was in losing crews in 2005 and 2006 before taking a year out to concentrate on his studies.
Oxford will field the oldest ever Boat Race competitor in American Mike Wherley, 36, a three-time world champion in eights and a veteran of the 2000 and 2004 Olympics.
Cambridge: Colin Scott 13st 8lbs, Tim Perkins 15st, Henry Pelly 13st 13lbs, Tobias Garnett 14st 10lbs, Peter Marsland 16st, Tom Ramsley 15st 11lbs, Tom Edwards 13st 11lbs, Ryan Monaghan 15st 3lb; cox - Rebecca Dowbiggin 7st 9lbs.
Oxford: Jan Herzog 14st 3lbs, Toby Medaris 16st 2lbs, Ben Smith 15st 5lbs, Aaron Marcovy 16st 1lb, Michael Wherley 15st 8lbs, Oliver Moore 15st 13lbs, Charles Cole 15st 1lb, William England 15st 4lbs; cox - Nicholas Brodie 8st 5lbs.