His move to the local rivals will heighten the tension between the two sides as the final games of the Celtic League take place and team jockey for seeding for the upcoming Celtic Cup.
"Strength in depth is important to build on the success of this year," said Ospreys manager Derwyn Jones. "Leigh has had some time away from the game, and has returned mentally and physically refreshed."
Jones added: "While Leigh has been away he has been monitored and working to a conditioning programme we agreed with him before he left. We look forward to him joining us for pre-season training, as Leigh is in better shape than he has been for many years.
"It is great that another local player has decided to join the Ospreys at what is an exciting time for the region. He will bring quality and experience to our young squad."
Davies started his first-class career in west Glamorgan with Neath RFC, his hometown club.