The left-handed Olsen is nursing back tendinitis in his throwing shoulder, while Mitre is recovering from tightness in his right forearm muscle.
In recent days they had been throwing off flat ground, and the mound work was their first since they were respectively shut down from Grapefruit League action.
Both felt fine afterwards, but time now becomes a concern with Opening Day set for March 31 against the Mets at Dolphin Stadium.
While manager Fredi Gonzalez has not said who will go on Opening Day, Olsen has been the most practical option based on his previous two years. Mitre, meanwhile, is viewed as one of Florida's top of the rotation starters.
Gonzalez said on Tuesday morning that both will throw off the mound again on Thursday.
"We haven't done anything past that," Gonzalez said regarding their respective time tables.
Olsen threw five scoreless innings in two Grapefruit League starts. His bout with tendinitis was revealed on March 5. Mitre felt forearm tightness during a 15-pitch first-inning outing on Feb. 28.
Of the two, Mitre is being groomed back more slowly.
If Olsen is to be part of the rotation by Opening Day, he would have to get back into games pretty quickly.
"It will be close," Gonzalez said.
Asked if Olsen could be ready by Opening Day, Gonzalez added: "It's conceivable, in my mind."
A year ago, the Marlins rotation was hit hard by injuries. Three of their projected starters when the '07 season opened were lost for substantial stretches. Josh Johnson, Anibal Sanchez and Ricky Nolasco each won 10 or more games as rookies in 2006, but in '07 they dealt with injuries.
Olsen has been one of the team's most durable starters the past two seasons, posting more than 30 starts in each of those years.
Mitre was shut down last September because of arm fatigue.
With Olsen possibly not being ready for the opener, Mark Hendrickson and Andrew Miller are candidates to take the ball in the opener, while the Mets will be going with Johan Santana.
Joe Frisaro is a reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.