Oakheart is in the Tree of Life. After you cleanse Wavebringer and Stoneheart(?), Oakheart will offer you a quest and then afterward, you will defeat him in battle. Then return to the Jade Palace where all three nature spirits will greet you and award you the Spirit Walker badge.
You have to start with the Mossback quests. First, its Cave of Solitude,Kishibe Village,Shirataki Temple,Ancient Burial Grounds,Village of Sorrow,and then tree of life. you have to do his quests in that order, and you fight oakheart at village of sorrow