The forward pair both picked up injuries in the win against Crawley last weekend that secured the Latics' npower League One status for another season.
Smith was forced off with an Achilles problem while Barnard suffered a hip injury and neither will recover in time for the final game of the campaign.
In the duo's absence, Lee Johnson's side fired a blank against Shrewsbury in midweek and also lost midfielder Jonathan Grounds to a neck complaint.
Grounds is rated 50-50 as Oldham look to end their campaign with a fourth win in five games.
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