Oogway joined up with Our Party in the Candlekeep. See C. Bane of the Kraken.
This humanoid-turtle (Tortle) had been a captive of the Candlekeep for many years, serving to pay off a late fee for a book he had (without authorization) "checked out".
Plea: "I'm a slow reader."
Worse, the enchantments of the book had been drained in the process of his apparently successful use of it to contact an entity he calls "Great A'Tuin" - who has become his warlock patron.
Oogway is evasive in sharing his past, but it seems he comes from far to the south, perhaps even beneath the Chult peninsula, if not from another world entirely - for his kind are essentially unknown in the lands of the Sword Coast. He keeps a familiar named (add-this) that most closely resembles a Unknown.