O’Brien missed last week’s win at Grimsby with injury, but the defender will return to the fray.
Matty Dolan, Joss Labadie and Robbie Willmott all suffered hamstring injuries at Blundell Park but should be fine to play.
New signing Emmanuel Osadebe is unlikely to play after he did not join from Cambridge in time. Crawley have doubts over midfielder Dannie Bulman.
Bulman, who came off during the second half of last weekend’s win over Barnet, is struggling with a knee injury and will be given a late fitness test.
Manager Harry Kewell will definitely be without midfielderJosh Payne as he will complete a three-game ban.
Southampton remain in the hunt for Red Bull Salzburg's Amadou Haidara and are reportedly readying a £15million offer.
Aston Villa are reportedly weighing up a summer move for out-of-favour Huddersfield defender Michael Hefele.
Southampton are lining up a summer bid for Huddersfield Town midfielder Aaron Mooy, reports claim.