Centre-back Olsson (calf) was forced off at half-time in Tuesday's 2-0 win at Swansea and is also a doubt for next Wednesday's home clash against Coventry.
Striker Bednar (thigh) and versatile loanee Reid (groin) missed the trip to the Liberty Stadium and will again sit out against North End, with the former likely to be out for at least a further fortnight.
There is some good news for the Baggies, however, with Jerome Thomas progressing well in his recovery from a hamstring problem and an outside bet to return against North End.
Fellow wide man Andy Slory is likely to be back in contention after missing the midweek win but Chile international Gonzalo Jara remains sidelined with the broken metatarsal that is likely to prevent him from playing again this season.
