O'Malley was a late withdrawal from the starting line-up for the Bank of Ireland Series clash with Northampton Saints last Friday due to illness.
Following scans at the weekend, Eamonn Sheridan's knee injury is worse than first thought. Ligament damage is likely to rule him out until the New Year.
Shane Jennings' call-up to the Ireland Rugby World Cup squad means that there will be 14 Leinster representatives on duty in New Zealand including Isaac Boss and debutant Sean Cronin, who both featured against Northampton at Donnybrook.
David Kearney successfully came through his first game of the season, while the squad to take on the Ospreys will also include Fionn Carr, who scored his first competitive try on Friday since his return to the province this summer.
Other Leinster Academy graduates in the mix are Jamie Hagan, Rhys Ruddock and Dominic Ryan. The team and replacements will be announced on Thursday afternoon.