O'Connor has resumed contact training, but Thursday's game looks like coming too soon for his return from a back injury.
Connacht Academy graduate Dermot Murphy makes his first appearance in a league squad this season.
The 21-year-old Galwegian impressed during the recent run of pre-season friendlies and will hope to add to his tally of caps, with Connacht looking to gain revenge for last December's 21-9 defeat at Cardiff City Stadium.
Gavin Duffy, Keith Matthews, Niva Ta'auso and Troy Nathan have all been included in the backs division, with the hope that they will overcome some niggling injuries in such a short turnaround from the weekend.
Connacht coach Elwood said: "Gavin has gone for a scan on his back, Keith Matthews has a dead leg, Niva (Ta'auso) is sore. All the forwards are sore.
"They took a battering (against Ulster) so you can bracket all of them. Troy (Nathan) got a knock on his back but should be okay. Outside of that we are fine.
"It's a big ask, but we want to get something out of every game we play and that's our objective again on Thursday."
CONNACHT Squad (v Cardiff Blues) - Backs (11): Cillian Willis, Frank Murphy, Miah Nikora, Ian Keatley, Fionn Carr, Keith Matthews, Niva Ta'auso, Aidan Wynne, Troy Nathan, Gavin Duffy, Darragh Fanning.
Forwards (14): Ronan Loughney, Brett Wilkinson, Dermot Murphy, Sean Cronin, Adrian Flavin, Rob Sweeney, Jamie Hagan, Andrew Browne, Michael Swift, Bernie Upton, Michael McCarthy, Ray Ofisa, Mike McComish, Johnny O'Connor.