The 28-year-old, who spent the first half of the campaign with Latics' npower League One counterparts Bournemouth, will move to Boundary Park subject to Premier League and Football League approval.
Barnard began his career at Tottenham and, following loan stints at Exeter, Stevenage, Leyton Orient, Northampton and Crewe, he came to prominence after joining Southend in January 2008.
A haul of 37 goals in 69 starts during a two-year spell with the Shrimpers earned Barnard his move to St Mary's.
In his first full season, he helped fire Southampton to promotion from League One but featured sparingly in the Championship last term and has not featured in their first team in the top flight.
Barnard has made 40 starts and 28 substitute appearances for the Saints, scoring 24 goals.
He is Oldham boss Paul Dickov's third loan signing of transfer deadline day following Watford forward Chris Iwelumo and Reading winger Jordan Obita to Greater Manchester.
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