The team's official Twitter account had the news:
Oswalt last pitched for the Texas Rangers in 2012, when he went 4-3 with a 5.80 ERA splitting time between the bullpen and the starting rotation. Dealing with injuries and unhappiness with his role in Texas, Oswalt was rarely used down the stretch and made his final appearance with the Rangers in their regular season finale against the Oakland Athletics.
The 35-year-old was granted free agency at the end of the 2012 season.
Career highlights for Oswalt include leading the NL with a 2.98 ERA as a member of the Houston Astros in 2006, finishing in the top six in Cy Young voting six different times and going 7-1 with a 1.74 ERA in 2010 after Houston traded him to the Philadelphia Phillies as part of a midseason deal that resulted in a trip to the NLCS.
For his career, Oswalt has a 163-96 record and a 3.28 ERA.
Colorado currently sits in first place in the National League West heading into Thursday night, holding a one-game lead over the San Francisco Giants and a two-game advantage over the Arizona Diamondbacks.
Although starter Jhoulys Chacin has been stellar with three wins in his first four starts, he has been out since mid-April with a back issue. Although he's expected to return soon (h/t Denver Post), no other regular-rotation mate has an ERA under 4.18.
As a team, the Rockies currently have a 3.72 ERA to go along with their 17-11 start to the 2013 season.
Oswalt will head to extended spring training to prepare for a potential stint in the major leagues, and with the Rockies suddenly a playoff contender after April, this is a move that could pay huge dividends down the road if another injury to a starter should occur.
However, don't expect Oswalt to make an immediate impact anytime soon.