O'Malley was activated from the 60-day disabled list and optioned to Triple-A Tacoma on Friday.
O'Malley had been out since early May while recovering from shoulder tendinitis, but performed well in 14 minor-league rehab games, hitting .311 with a .986 OPS over 45 at-bats. He'll return to Tacoma for now where he started the year, serving as organizational depth in the event of an injury. O'Malley hit just .229/.299/.319 with the Mariners last year and missed the beginning of the 2017 season after undergoing an appendectomy.