O’Brien and Ryan Tunnicliffe come into Danny Cowley’s team from Crewe, with Denver Hume and Joe Morrell dropping to the bench.
Ipswich also make two switches from their mid-week success at Lincoln.
Former Pompey favourite Conor Chaplin starts with Cameron Burgess, as Sone Aluko drops the bench with the injured George Edmundson missing out.
Michael Jacobs fails to make the squad with the summer Ipswich target due to return to training this week.
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Pompey: Bazunu, Carter, Raggett, Robertson, Romeo, Thompson, Tunnicliffe, Ogilvie, Curtis, O’Brien, Hirst. Subs: Webber, Vincent, Hume, Mingi, Morrell, Jewitt-White, Walker.