The 29-year-old has scored 59 goals in 275 appearances for the O's, with five of those games coming this campaign.
But last term he suffered a season-ending injury in November and it was reported the O's were willing to sell him in the transfer window.
A statement said: "We'd like to thank Dean for his service to the club and wish him all the best in the future."
Winger Dean Cox has left League Two side Leyton Orient after his contract was terminated by mutual consent.