Oakland starter Jarrod Parker (10-6) won his eighth consecutive decision and extended his unbeaten streak to 16 starts, holding the Orioles to one run on five hits over eight innings.
Tillman held the A's to just one hit over his first five innings before Oakland tied the game at 1 in the sixth on Jed Lowrie's two-out RBI double.
Despite the no-decision, Tillman continued his dominance against the AL West. He has a 12-1 record and 2.41 ERA against teams from that division, and he's pitched to a 1.96 ERA in three career starts against Oakland covering 18 1/3 innings.
Tillman mixed his fastball, curveball and changeup well Saturday and seemed to get stronger as the game went on, retiring 14 of the final 16 batters he faced, including seven strikeouts in that span.
"I feel like once I'm able to get in a groove, I feel better towards the end of the game," Tillman said. "The deeper I get, the better I feel, so as long as I'm able to keep my offspeed pitches there, both of them, I can stay in the groove throughout the game."
Tillman was at 103 pitches after seven innings, but Showalter kept him in the game for the eighth, and the right-hander struck out the first two batters of the inning on his way to retiring the A's in order.
"When you go back out and pitch the eighth inning like he did, that's impressive," Showalter said. "Parker matched him, which they needed. … But no, he emptied the tank there in the eighth inning."
Tillman made a fourth unsuccessful attempt at winning his 15th game of the season. The Orioles haven't had a 15-game winner since Erik Bedard in 2006.
"Not worried about that," Tillman said. "I'm more frustrated that today's a loss. It's a frustrated clubhouse. We've got another one tomorrow. We've got a chance to win the series. That's big. We'll look forward to it tomorrow."
The Orioles' only run came on Ryan Flaherty's eighth homer of the season, a shot to right field that ended Parker's run of seven retired batters to open the game.
The Orioles were 0-for-6 with runners in scoring position. In the sixth and eighth innings, they stranded the go-ahead run at third base. They opened both innings with a leadoff hit and had runners at the corners both times but couldn't score.
In the sixth, the Orioles were unable to take advantage of Manny Machado's leadoff double.
Nate McLouth opened the eighth with a leadoff single and moved to second on Machado's sacrifice bunt. But after the A's intentionally walked Chris Davis, Adam Jones hit into a force out at second and Matt Wieters grounded out to end the inning.
The Orioles also had runners at first and second with two outs in the fifth but couldn't score.
"He works in and out," Jones said of Parker. "He has a good changeup. Just kept us off balance. ... You got to tip your cap to both pitchers. I think that was a well-pitched ball game on both sides. Obviously a 2-1 game means a well-pitched ball game on both sides."
Closer Grant Balfour retired the Orioles in order in the bottom of the ninth for his 32nd save.