One of the greatest performances of O'Leary's career and in Boston Red Sox postseason history took place on October 11, 1999 in the decisive game 5 of the 1999 American League Division Series versus the Cleveland Indians. Boston entered the top half of the third inning trailing 5-2. With runners on second and third and first base open, Indians manager Mike Hargrove chose to intentionally walk Nomar Garciaparra, who had homered in the first inning, to load the bases for O'Leary. O'Leary responded by sending a Charles Nagy breaking ball over the right field fence for the first grand slam in Red Sox post-season history. When the seventh inning began, the game was tied 8-8. Again, Hargrove chose to intentionally walk Garciaparra to get to O'Leary, this time with Paul Shuey on the mound. O'Leary responded by hitting a line drive over the right field fence for a 3 run home-run. O'Leary's 7 RBI, along with 6 no-hit relief innings from Pedro Martinez, helped seal the victory and advanced the Red Sox to the 1999 American League Championship Series against the New York Yankees.