Dawson suffered serious ankle ligament damage in last week's draw at Ipswich and has been ruled out for up to eight weeks, so Kelvin Etuhu, who replaced the Irishman at Portman Road, is hoping to keep his place.
Winger O'Brien missed out last week after sustaining a dead-leg in the recent win over Leeds and is rated 50/50.
Defender Jim McNulty was also absent against Ipswich after tweaking a hamstring in training and is still struggling.
Manager David Flitcroft, who has won two and drawn one of the four games in which he has taken charge since Keith Hill's sacking last month, must decide whether to leave striker Danny Rose on the substitutes' bench.
Teenager Rose, who has scored twice in his last three appearances, is pushing for only his third start for the Reds and his first of the season.
Defender Bobby Hassell is also pushing for a recall to the side.