Straily (6-2) held the hosts to two hits and won his fifth consecutive division while leading the Athletics to their 10th win in their last 13 games.
Brandon Moss drove in both Oakland runs with home run in the fourth inning.
Pittsburgh lost its fourth in a row despite a solo home run by Pedro Alvarez and a solid start by Gerrit Cole (4-2). Cole allowed five hits and two walks and struck out seven innings.
The Pirates put the tying run on first in the bottom of the ninth, but Balfour struck out the next two batters and retires Russell Martin on a ground ball to save the win.