Oak’s Amulet is a quest item acquired after defeating Oak in the Wetlands. It is only obtainable during one of the bandit faction quests in Act 2 that requires the character to oppose Oak: “Deal with the Bandits”, “The Bandit Lord Alira”, or “The Bandit Lord Kraityn”.
When combined with Alira’s Amulet and Kraityn’s Amulet, The Apex is formed.