Tettleton was given the starting catcher's job for the 1986 season, but his offensive statistics failed to impress. He posted a .204 batting average in 90 games, along with 10 home runs and 35 runs batted in. His defensive abilities were termed as adequate. By the end of the season, Athletics manager Tony La Russa had Tettleton platooning with the left-hand hitting catcher, Jerry Willard. Tettleton began the 1987 season as the Athletics starting catcher. However, injuries and weak hitting limited him to 82 games, as newcomer Terry Steinbach began to make a positive impression. He ended the year with a dismal .194 batting average. On March 28, 1988, the Athletics released Tettleton during spring training. On April 5, he signed a contract to play for the Rochester Red Wings in the Baltimore Orioles organisation.
Tettleton started the season in Rochester but was soon called back to the major leagues, where he shared catching duties with Terry Kennedy in 1988, and improved his batting average significantly to .261. 1989 was a breakout year for Tettleton as he started the season hitting 13 home runs in April and May, setting a new major league record for catchers. He hit 20 home runs by midseason, earning him a place as a reserve player for the American League team in the 1989 All-Star Game. In August, Tettleton had to undergo surgery to remove a cyst from his left knee and missed almost one month. He finished the season with a .258 batting average, 26 home runs, 65 runs batted in, and an impressive .369 on-base percentage, helping the Orioles improve from last place in the American League East in 1988 to second place in 1989, two games behind the Toronto Blue Jays. Tettleton's hitting performance earned him the 1989 American League Silver Slugger Award, which is awarded annually to the best offensive player at each position.