Payne retired injured at half time of the Celtic League game v Cardiff Blues last Saturday while O'Callaghan was injured in training prior to the Llanelli game at the start of this month.
The effects of a high tackle in that Cardiff game rules Payne out and O'Callaghan, who hasn't played a full game since the Celtic League encounter against Connacht last April, tore a calf muscle in training.
Coach Alan Gaffney expects to finalise his starting line-up on Thursday.
P Burke, C Cullen, P Devlin, J Kelly, R Henderson, A Horgan, J Holland, M Lawlor, R O'Gara, M Prendergast, P Stringer, J Storey. T Bowman, A Foley, J Flannery, M Horan, T Hogan, J Hayes, S Keogh, D Leamy, G McIlwham, P O'Connell, A Quinlan, F Sheahan, J Williams, D Wallace.