Birtsas appeared in four games as a minor leaguer for the Oakland A's before receiving a promotion to the big league club. With the A's already losing 10-0 to the Boston Red Sox, Birtsas made his major league debut on May 3, and pitched the final two innings without allowing any more runs while striking out three. After finishing two more games without allowing any additional runs to score, Birtsas made his first start on May 23, against the Baltimore Orioles. He allowed two runs in six innings while striking out five to pick up the win. He was added to the starting rotation shortly afterwards, and had a 9-2 record following a victory over the Mariners in Seattle on August 10. From there, Birtsas went 1-4 with a 6.10 ERA in his next eight starts to finish the season at 10-6 with a 4.01 ERA. Birtsas was slated to be one of two left handers out of the Athletics' bullpen for the season. He allowed five earned runs on two hits and three walks in just one inning in his first appearance of the season. Knee problems were cited as a potential cause of his ineffectiveness. After one more appearance, he was optioned down to triple ATacoma to "get his groove back." He spent the rest of the season at Tacoma, going 3-7 with a 5.07 ERA, used mostly as a starter. After splitting the season between Tacoma and double A Huntsville, Birtsas was again packaged with José Rijo, this time to the Cincinnati Reds for Dave Parker.
Cincinnati Reds
Birtsas entered his first Spring with the Reds competing for a long reliever/spot starter job. He was reassigned to the triple A Nashville Sounds toward the end of Spring training, but was up in the majors by the middle of May. With his record standing at 1-3 with a 4.50 ERA, he was optioned back to triple A at the end of August, but an injury to Rijo helped keep him in the majors through the rest of the season. Despite the turmoil the Reds faced in with manager Pete Rose's betting fiasco, Birtsas managed to put together a respectable season. He made a career high 42 appearances, and went 2-2 with a 3.75 ERA while earning his only career save, a rare four inning save, against the San Francisco Giants on August 7. He also got his only career hit in eighteen career at-bats on July 2, a home run off the New York Mets'Sid Fernandez. With new manager Lou Piniella in place for, the Reds started the season off with a nine-game winning streak, and remained in first place in the National League West for the entire season. Four of these nine games were won by relief pitchers, who also compiled five saves. The relief corp of the Reds earned the nickname "The Nasty Boys" along the way, and lived up to this reputation by leading the National League with 46 saves, and compiling 385 strikeouts. On June 4, Birtsas accomplished the rare feat of striking out four batters in one inning. Regardless, following a poor performance against the Atlanta Braves on June 20, Piniella seemed to lose faith in Birtsas. Birtsas saw little work in key situations in the second half of the season, and was optioned down to triple A toward the end of July. He was recalled in September, and was part of the Reds' post-season roster, but did not make an appearance in the 1990 National League Championship Series with the Pittsburgh Pirates or World Series against the A's. The Reds were 0-12 in Birtsas' final twelve regular season appearances.
In 1992 Birtsas pitched for the Rimini Pirates in the Italian Baseball League, and won two games of the championship series, including a shutout in the third and decisive game, leading his team to the league title with a 3-0 sweep. After pitching in Italy, Birtsas retired due to hip problems.
