O'Callaghan retains the number 4 jersey having come on in Llanelli as a 55th minute replacement for Donncha Ryan, who is listed this weekend among the replacements.
Earls injured a knee in the opening minute of the league derby against Leinster on November 4 and did his first full training spin late last week before coming through two sessions this week with flying colours.
It is not surprising that Leamy will fill the number 6 shirt after an impressive showing in the second half last weekend when he came on for O'Mahony.
Tommy O'Donnell fills the gap on the bench created by Leamy's promotion and Stephen Archer has been preferred as loosehead prop cover for BJ Botha.
Young scrum half Conor Murray is looking forward to playing in front of a packed Thomond Park, but he is not taking anything for granted.
"Yeah, I love playing there, the atmosphere on days like this is brilliant. But as Tony (McGahan) said, this is just the second half," he said.
"And the result will be decided by what happens between the touchlines. It's going to be every bit as close and tense as it was last weekend."
MUNSTER (v Scarlets): Denis Hurley; Johne Murphy, Keith Earls, Lifeimi Mafi, Simon Zebo; Ronan O'Gara, Conor Murray; Wian du Preez, Damien Varley, BJ Botha, Donncha O'Callaghan, Paul O'Connell (capt), Denis Leamy, Niall Ronan, James Coughlan.
Replacements: Denis Fogarty, Marcus Horan, Stephen Archer, Donnacha Ryan, Tommy O'Donnell, Tomas O'Leary, Ian Keatley, Danny Barnes.