O'Brien has been struggling with a thigh strain since the start of this month, while Bruce was forced off after 11 minutes in the home win over QPR due to a hamstring injury.
Leigh Bromby could be given an extended run alongside Neill Collins in the side after stepping off the bench to replace Bruce last week.
Simon Grayson, who has no other injury concerns, is not expected to make any unforced changes as his side look to extend their unbeaten league run to 10 matches, but the likes of Davide Somma, Billy Paynter, Ross McCormack, Lloyd Sam and Amdy Faye are all pushing for a place in the starting line-up.
Source: PA