Fairmont grad Makayla Waterman had six points during the spurt, which Calhoun capped with a jumper at the 1:13 mark.
After shooting 59 percent in the first quarter, the Terrapins went 4 for 17 in the second (24 percent).
Ohio State shot 67 percent in the second quarter and had 16 rebounds to Maryland’s eight.
Mitchell, Calhoun and Linnae Harper had 10 points apiece in the first half to lead Ohio State while no Terrapins reached double figures.
Ohio State led by as many as 18 in the third quarter, but the Terrapins closed the stanza on a 10-2 run to draw within 57-49 heading into the fourth.
After each team made a pair of free throws to start the last quarter, Maryland went on an 8-2 run to cut Ohio State’s lead to 61-59 with 5:58 to go.
That prompted Ohio State coach Kevin McGuff to call timeout.
Mitchell made a step-back 3 coming out of the timeout and hit another shortly thereafter to make it an eight-point game with under three to go.
Maryland never recovered.
Ohio State won the Big Ten women’s basketball tournament for the sixth time, the last coming in 2011.
The Terps entered the game undefeated in Big Ten tournament play since joining the league in 2015. That included winning the ’15, ’16 and ’17 conference tourneys.
The Buckeyes entered the game projected by ESPN to be a No. 3 seed in the NCAA tournament.
That would likely mean they would host first- and second-round games at St. John Arena.
The Final Four will be held in downtown Columbus at Nationwide Arena.