O'Sullivan signed a minor-league contract with the Nationals on Sunday.
O'Sullivan will return stateside after opening the season with the Nexen Heroes of the Korean Baseball Organization. Nexen granted O'Sullivan his release in May after the right-hander served up 14 runs over eight innings, but it appears the Nationals think he could still fortify the team's pitching depth at Triple-A Syracuse. O'Sullivan last appeared in the big leagues in 2016 with the Red Sox, making five appearances and turning in a 6.75 ERA.