Wesley, 30, was cut by the Chiefs last week after a year of acrimony. Wesley wanted out of Kansas City since before the 2007 season and was nearly traded to the Chiefs' AFC West rival Denver Broncos.
Denver expressed some interest in Wesley in recent days but he ended up signing with Oakland after visiting the team Monday.
Agent Drew Rosenhaus said Wesley was happy to remain in the AFC West and looked forward to playing against the Chiefs twice this season.
Wesley has 29 career interceptions, along with six sacks, 12 forced fumbles and four fumble recoveries.
Oakland reports to training camp in Napa on Wednesday with the first practice being scheduled for Thursday.
Bill Williamson covers the NFL for ESPN.com. Information from The Associated Press was used in this report.