Measure O is a ballot measure that passed in 2006 and established Ranked Choice Voting in Oakland.
http://ballotpedia.org/City_of_Oakland_Instant_Runoff_Voting,_Measure_O_(November_2006)
http://archive.fairvote.org/oaklandirv/webarchive/irvnews.html
http://journalism.berkeley.edu/projects/election2006/2006/11/oakland_adopts_instant_runoff.php