O'Day, 29, was 0-1 with a 5.40 ERA in 16 games and 16 2/3 innings this season. He also was 1-0 with a 2.53 ERA, four walks and 27 strikeouts in 18 games and 21 1/3 innings with Double-A Frisco (one game) and Triple-A Round Rock (17).
O'Day, a submarine-stylist similar to Chad Bradford, also has pitched for the Angels in 2008, the Mets in 2009 and the Rangers in 2009 and 2010. He's 8-5 with a 2.89 ERA and 148 strikeouts in 180 2/3 major league innings. Right-handers are batting .214 against him in 471 plate appearances.
The Rangers outrighted O'Day to remove him from the 40-man roster.
The Orioles also reinstated pitcher Jake Arrieta, second baseman Brian Roberts and outfielder Luke Scott from the 60-day disabled list. They have 38 players on their 40-man roster.
NOTE: As expected, catcher/infielder Jake Fox refused his outright assignment and became a free agent. Pitcher Chris Jakubauskas made the same decision over the weekend.