Announced on Wednesday night, O'Farrell is the last of the main party candidates to be picked for the contest on February 28, which was triggered when Liberal Democrat ex-Cabinet minister Chris Huhne resigned in disgrace.
The Lib Dems are favourite to retain the seat, which Huhne secured at the 2010 general election with a majority of 3,864. Local councillor Mike Thornton is representing the party.
However, the Tories have signalled they are setting aside coalition links to mount a fierce campaign.
Earlier today, Ukip candidate Diane James said Romanians were associated with crime and called for a temporary halt to all immigration.
James was in the Hampshire constituency for a rally with supporters aided by Ukip leader Nigel Farage.