Tucker was named the American League Player of the Month on Monday, and O'Neill took National League Player of the Month honors.
Tucker was the best hitter down the stretch for the AL West-champion Astros. Over the season's final month, the 24-year-old right fielder batted .346/.438/.692 with eight home runs, 19 RBIs, 20 runs scored and an 1.130 OPS.
Tucker's slugging percentage and OPS during that time led the AL, and his batting average and on-base percentage both ranked second.
O'Neill's breakout 2021 season helped carry the Cardinals to a 17-game winning streak in September and an NL Wild Card berth. The 26-year-old left fielder batted .328/.377/.731 with 13 home runs, 30 RBIs, 31 runs scored and an 1.108 OPS in September and October.
O'Neill led the National League in home runs, RBIs, runs scored and slugging percentage for that period, and was also the Major League leader or co-leader in all those categories.
It's the first Player of the Month Award for both Tucker and O'Neill. The Astros' most recent previous Player of the Month was Alex Bregman in August 2019, and the Cardinals' latest Player of the Month before O'Neill was Paul Goldschmidt in July 2019.