Jacksonville released Henderson in late April just days after the NFL draft.
Oakland had been looking for another defensive tackle after releasing Gerard Warren this offseason.
Henderson, an eight-year veteran, was pursued by a handful of teams.
Henderson, 31, was regarded as one of the best players remaining in the unrestricted free-agent pool. The two-time Pro Bowl defender was released as the Jags continued in the offseason to remake a defense that ranked No. 23 in the league in 2009.
A former University of Tennessee standout, Henderson started in 120 of 124 appearances with the Jaguars. He registered 329 tackles, 29 sacks and eight forced fumbles. But he did not have more than three sacks in any season over the last five years.
Information from ESPN NFL Insider Adam Schefter and ESPN.com senior writer Len Pasquarelli was used in this report.