I've found a number of my life friends getting stuck when they get their quest to see Oakheart. They go to see him and the light's on but nobody's home. A friend of mine Suri Spiritcaller recently got stuck on this as well and found the answer online. I'm going to post her solution as she sent it to me as it is pretty involved.
Oakheart Side Quests
This is a guide about the Oakheart side quest in MooShu.
You have to complete this in a certain order for it to work.
First you need to complete Mossback's quests in order.
(Cave of Solitude) Quest: Collect 6 rocks from Corrupted Earth Spirit
(Kishibe Village) Quest: Defeat 10 Diseased Water Elementals at Shirataki Temple
(Shoshun Village) Quest: Defeat and talk to Great River Spirit
(Ancient Burial Grounds) Quest: Collect 10 Corrupted Earth (Can only be found outside the houses in the Burial Grounds); Defeat Crazed Forest Spirit. After you do those quests for Mossback, go to the Village of Sorrow and talk to Wavebringer.
(Village of Sorrow) Quest: Collect 10 Tainted Bark from Befouled Woodwalker. Defeat and talk to Oakheart. Go back to Wavebringer. After that, you can go to the Tree of Life instance and do Oakheart's quests.
(The Tree of Life) Quest: Collect 3 Pure Seeds from Kogani Death Crawlers Plant, 3 Fertile Dirt, go back to the Jade Palace and go into the dojo closest to the entrance to Shoshun Village and talk to Oakheart.