The full-back/stand-off missed last week’s home defeat by Wakefield Trinity because of concussion, but has been given the all clear and insisted: “I’m fine.
“I’ve been through the usual protocols and I’m used to it by now.
“It was more frustrating, but in rugby league these things happen.
“I’ve had no recurring symptoms, I’ve been through all the return to play protocols and seen a specialist and I am good to go.”
O’Brien’s latest head knock was caused by an accidental clash of heads in Tigers’ win over Hull.
He stressed: “I won’t change how I am playing.
“I’d like to think I put my body on the line and sometimes you come off second best unfortunately.
“That’s just how the game goes.”
Tigers were beaten 32-6 by Trinity and Saints are also on the back of a defeat, 44-12 at Salford Red Devils.
“We weren’t at the races,” O’Brien admitted of Tigers’ derby setback.
“We’ve reviewed it and worked hard in training this week and we’re looking forward to giving it a crack on Sunday.
“There’ll definitely be a response from them, but we are looking to respond as well.
“It will be two teams itching to get at each other and get the two points.”
Sosaia Feki has been included in Tigers’ initial squad for what would be only his second appearance since joining the club ahead of the 2020 season.
Alex Mellor is available after concussion and Cheyse Blair, Brad Martin, Cain Robb and Sam Hall are also in contention.
Jake Mamo is ruled out because of concussion and Daniel Smith and George Lawler also drop out from last week’s team.
Tommy Makinson and Dan Norman could return for Saints after injury and Sam Royle has been recalled from loan at Hull KR, but Agnatius Paasi (concussion) drops out.
St Helens: from Welsby, Makinson, Lomax, Walmsley, Roby, Lees, Batchelor, Knowles, Lussick, McCarthy-Scarsbrook, Sironen, Wingfield, Bell, Davies, Hurrell, Norman, Royle, Bennison, Baxter, Hill, Pemberton.
Castleford Tigers: from Olpherts, Faraimo, Richardson, Watts, McShane, Edwards, Milner, Westerman, Griffin, Fonua, Sutcliffe, Eden, Blair, Matagi, Martin, Hall, Feki, O’Brien, Robb, Qareqare, Mellor.
Referee: Chris Kendall (Huddersfield).
Kick-off: Sunday, 1pm.