Right-hander Daniel Mengden, who was 1-7 with a 6.39 ERA entering the game, silenced the Royals on three hits through seven innings. He walked none and struck out six.
The loss reduced the Royals' magic number to be eliminated from Wild Card contention to 12, pending the Blue Jays-Angels outcome. Toronto holds the second Wild Card spot in the American League.
The series provided the best run differential (31) in a four-game series in Oakland history (previous mark was 27 against Seattle on April 17-19, 1981). The Royals also set a record for most runs allowed in a four-game set with 43.