The 33-year-old stopper was released by United at the end of last season. As we reportede he has been very critcical of Fergie's treatment of him.
Osasuna are not renewing keeper Ricardo Sanzol's contract, so Ricardo will compete with current first choice Juantxo Elia for the number one jersey.
They qualified for next term's Uefa Cup as runners-up in the King's Cup.
Osasuna sporting director Juan Jose Lorenzo: "A good keeper like Richard Sanzol is leaving but we are getting a good goalkeeper and a great guy in Ricardo."