Oskill

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.

Otherwise, oskills function as they would for their native class. They can be assigned to left-click and right-click buttons as well as hotkeys, if the skill normally allows it.

They only appear on specific set bonuses, unique equipment, and Rune words.

Same class cap
Oskills are capped to +3 skill levels if used by their native class. For example, the four-piece set bonus for Trang-Oul's Avatar is +10 to. Classes other than Sorceress will be able to cast a level 10 Meteor (or higher, with + to all skills) while wearing four pieces of Trang-Oul's. A Sorceress wearing the same will receive only +3 to her existing Meteor skill, however. This allows oskill bonuses to be large enough to be useful to other classes without being overpowered for their native class.

Examples
A common application of oskills is switching to a weapon, using the Battle Command and Battle Orders oskills granted by Call to Arms, then switching back to the main weapon, retaining the increased life and mana pools from Battle Orders. This technique is especially potent for characters with many +skill bonuses, such as Sorceresses and other spellcasting based builds.

Another common application of an oskill is armor, which allows any class to cast.

Oskills are the basis for some unusual builds, such as a Sorceress specializing in Enchant using a weapon's Zeal oskill to deliver fast melee attacks, or a  specializing in normal (non-skill) weapon attacks with one weapon using a  weapon's Werebear and Lycanthropy oskills to increase defense and maximum life.

Table of oskills
* capped at +3 for native class