The Mariners, who traded Jeuris Familia to the Reds in exchange for a draft pick in 2020, will send Oh to Seattle to be the first player from their organization traded to another team in franchise history.
The Mariners acquired Familia in a July 24 trade with the Red Sox that sent outfielder Adam Lind to Seattle.
The two teams have been linked for a while now, with the Mariners making it official that they were going to make the deal official.
They traded Lind for the 2019 first-round pick and a conditional second-round draft pick on Thursday.
Oh is expected to sign with the Reds as soon as Friday.
Oh has played in two of the Mariners’ first three games, but has yet to record an at-bat.
He has five home runs and 15 RBIs in 46 games with Seattle.
Oh could see his role expand in the outfield.
The 29-year-old outfielder has hit .271 with seven home runs, 29 RBIs and 11 stolen bases in 122 games with the Diamondbacks and Dodgers this season.
The Red Sox are in need of a shortstop.
Right fielder Brandon Crawford will likely miss the rest of the season with a strained left hamstring.
Crawford, 28, hit .273 with five home run and 24 RBIs this season with Chicago.
He’s batting .247 with seven homers and 25 RBIs with Los Angeles.
Right-hander Brad Ziegler (knee) is out indefinitely with a right oblique strain.
Righty Nick Tepesch (back) will be out for the rest for at least a month with a sprained right ankle.
They will likely start with Brad Mills as the designated hitter.
Mills is hitting .287 with seven doubles, three home runs in 32 games for the Mariners this season and has five saves and a 1.92 ERA.
Mills will start for the Diamonds on Sunday against the Mets.
The Braves, meanwhile, are still trying to come to terms on a contract extension with second baseman Miguel Andujar.
They have been without Andujaas bat for the past few days with a lower-body injury.
They are hopeful to have a deal signed sometime in the next couple of weeks.
The 28-year to the Braves was one of the most exciting and promising prospects in baseball last year.
He hit .295 with nine home runs as a rookie in 2016 and has a .291 average in his last 33 games.
He is batting .302 with 12 home runs this season, including four homers in the last six games.