The 30-year-old had been a free agent since leaving Columbus in November following seven seasons with the Crew. O'Rourke has appeared in 188 MLS matches, including 22 games (19 starts) last season.
The Ohio native, who started every playoff match during Columbus' run to the 2008 MLS Cup title, also had stints earlier in his career with San Jose and New York.
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After conceding just 33 goals throughout the 34-game regular season in 2013, the 2-3-7 Timbers have already allowed 20 goals through 12 matches this season.
The team's center backs have been the subject of some criticism, with offseason acquistion Norberto Paparatto struggling for minutes and holdovers Pa Modou Kah and Futty Danso delivering inconsistent performances.