O'Connell was not named in the extended squad announced on Wednesday. The Munster management waited to see how the province's regular captain came through what was only his second full contact session on Thursday since picking up the ankle injury against Leinster a month ago.
As it transpired, O'Connell came through training showing no ill-effects and has been included on a particularly strong looking bench alongside Marcus Horan, John Hayes and Peter Stringer, who between them have over 600 Munster caps.
Commenting on the O'Connell decision, head coach Tony McGahan said: "We thought it prudent to omit Paul from the original squad until we saw how he came through what was only his second serious workout (yesterday) since sustaining the injury.
"He reported no ill-effects today so we're delighted to be able to name him in the matchday squad."
When he sustained the injury in the April 2 encounter, it was expected that his reappearance would likely be in the latter stages of the Magners League campaign and McGahan was quick to heap praise on his backroom team and the player himself.
"I think the Munster medical and fitness staff deserve huge credit for having him back in the matchday squad at this stage," he explained.
"But it also demonstrates Paul's professionalism, dedication to his sport and commitment to Munster."
MUNSTER (v Harlequins): Felix Jones; Doug Howlett, Lifeimi Mafi, Paul Warwick, Keith Earls; Ronan O'Gara (capt), Conor Murray; Wian du Preez, Damien Varley, Tony Buckley, Donncha O'Callaghan, Mick O'Driscoll, Denis Leamy, David Wallace, James Coughlan.
Replacements: Mike Sherry, Marcus Horan, John Hayes, Paul O'Connell, Donnacha Ryan, Peter Stringer, Sam Tuitupou, Johne Murphy.