An oskill in Diablo II is a bonus to a specific class skill with no class restriction, which allows characters to use an oskill even if it is not normally available to their class. Oskills were added in the Lord of Destruction expansion. Oskills do not appear in the character's skill tree, and a character cannot spend skill points to improve oskills. Oskill levels are improved only by "+ to all skills" bonuses. If the character does not have the skill, the base level for that skill is considered 0. They only appear on specific set bonuses, unique equipment, and Rune words.