In Ocarina of Time, the ancient tree that watches over Kokiri Forest is the Great Deku Tree.
The Great Deku Tree is the guardian of the Kokiri, a childlike forest-dwelling people who live alongside fairy companions. He summons Navi and sends her to awaken Link, beginning Link’s journey beyond the forest.
At the start of the game, the Great Deku Tree has been cursed by Ganondorf, who wants the Kokiri’s Spiritual Stone of the Forest. Link enters the tree as the game’s first dungeon and defeats the parasitic monster responsible for the curse, but the tree dies soon afterward. Before his death, he gives Link the Kokiri’s Emerald and directs him toward Hyrule Castle.
A common mix-up is the Deku Sprout. The sprout appears seven years later and is the Great Deku Tree’s successor, not the ancient guardian himself. The Great Deku Tree also appears in later Zelda games, including The Wind Waker, Breath of the Wild, and Tears of the Kingdom.