Doncaster have made forward Dany N'Guessan their sixth summer signing.
Tottenham have confirmed that Romanian defender Vlad Chiriches has completed a move to Napoli.
The Greek outfit confirmed the capture - on a two-year contract - on their official website.
The 34-year-old former Barcelona full-back only signed a one-year contract extension at Monaco on Thursday after arriving in the principality the previous summer.
Abidal, who has won 21 titles including two Champions League winners' medals during a glittering career, recovered from a liver transplant in 2012 to resume playing for Barcelona.
The Frenchman, who has also played for Lille and Lyon, is Olympiakos's highest-profile signing of the close season so far as they look to build a squad capable of retaining their domestic crown and making a sustained challenge in the Champions League.