After progressing from 60 feet to 90, and eventually 120 feet, both Marlins' starters were given a thumbs up to take their throwing routine to another level.
So on Tuesday morning, both will throw their first bullpen sessions since their respective injuries.
Mitre has been nursing tightness to his right forearm muscle since Feb. 28, while Olsen has been dealing with a bout of left shoulder tendinitis since March 5.
Of the two, Olsen is considered more likely to return to the rotation either in time or closely after the regular season opens on March 31 against the Mets at Dolphin Stadium.
Before feeling some discomfort in his shoulder in the middle of last week, Olsen was regarded as the front-runner to be the Opening Day starter. Gonzalez has not yet named his starter for March 31.
"They are progressing to the mound, and that's a good sign," Gonzalez said.
Olsen has made two Grapefruit League starts, and he has compiled five innings total. The 24-year-old left-hander has not allowed a run thus far, and he's struck out three, while not surrendering a walk. He has hit one batter with a pitch.
Mitre, meanwhile, made one Spring Training start, on Feb. 28 against the Orioles in Fort Lauderdale, Fla. He faced three batters and was lifted after 15 pitches that day because his forearm felt tight.
The two entered Spring Training as front-runners to be among the club's top three starters.
Joe Frisaro is a reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.