Grandfather Oak is the oldest tree in the forests surrounding the Farm.
Grandfather Oak is one of the few Fables native to the Mundyworld. He is the patriarch of all trees surrounding the Farm and according to Geppetto in others beyond. He is deemed to be a powerful Fable, with Geppetto suspecting that what little magic that exists in the Mundyworld flows directly to him adding to his power.
After Geppetto escapes imprisonment brought onto him by animals on the Farm such as Reynard and King Noble, he wanders deep into the forest. There he discovers Grandfather Oak, and seeing the abundant arboreal magic the tree contain, sought his aid. Grandfather Oak agreed to an alliance and provided Geppetto with two dryads, Prince Aspen and Princess Alder, to be his bodyguards.
Later it was revealed as part of their bargain for providing the woodcutter his protection, Geppetto would provide a fresh sapling from the Enchanted Grove so that Grandfather Oak may sip at its power and that it would couple with his children. In doing so their alliance would secure their houses and together they would create a new wooden army that would conquer the Mundyworld.
In Farewell, three centuries after the showdown between Snow and Rose, Grandfather Oak is now working in cohort with Geppetto who has been planting satellite groves of his sapline across all forests in the world.