O'Day had been out since April 27 with a partial tear of the labrum in his left hip. He compiled a 2.45 ERA in eight relief appearances for Texas before the injury.
After recovering from surgery on the hip, O'Day had a 3.68 ERA in six minor league relief appearances leading to his return to the Rangers roster.
To create a spot for O'Day on the 25-man roster, the Rangers optioned left-hander Michael Kirkman to Triple-A Round Rock.
The additions of O'Day and right-hander Tommy Hunter, who was activated from the disabled list on Friday, will give the bullpen a boost, but Rangers manager Ron Washington said the Rangers are not done seeking relief help.