It's O'Brien's first start for over nine months and he'll line up beside Josh Van Flier (FLEE-ER) and Jamie Heaslip in the back row
Cian Healy, James Tracy and Mike Ross all come into the front row while Mike McCarthy has recovered from a foot injury to takes his place beside Devin Toner in the second row.
Meanwhile, Connacht coach Pat Lam has made seven changes to his team to face Zebre in Italy on Sunday.
Tiernan O'Halloran is injured so Cian Kelleher starts at fullback.
Stacey Ili is on the wing while Peter Robb joins up with Craig Ronaldson in midfield .
Kieran Marmion replaces Caolin Blade at scrum half
Hooker Shane Delahunt starts for the first time this season and packs down in the front row with the incoming JP Cooney and Finlay Bealham who shifts over to the tighthead side.
Quinn Roux returns having missed out last week due to injury while Sean O'Brien makes his first start of the season in the back row.
