Oskill items supply skill level bonuses to specific skills but without class restriction, such as +1 to Teleport on Enigma body armor. Any class or mercenary equipping oskill items is thus granted specific skills which are normally restricted to a certain class.
Mercenary cannot use oskills that are active skills, but they can use those that are passive skills.
Oskills receive passive bonuses including synergy bonuses, but they do not apply bonuses to other skills.
Oskill level can be increased by equipping multiple items supplying the same oskill, bonuses to All Skills, to Fire Skills (if oskill is a Fire Skill), or class-specific bonuses to the same skill: Teleport oskill from Enigma will benefit from a staff supplying +(1-3) to Teleport (Sorceress Only), even if the equipping class is not a Sorceress.
Oskill level bonus displayed is capped at +3 as viewed by the class to which the skill normally belongs: an Insight polearm/staff supplying a bonus of +4, +5 or +6 to Critical Strike displays only +3 to Critical Strike when viewed by an Amazon.
Oskill level bonus applied is capped at +3 total when equipped on the class or mercenary to which the skill normally belongs, regardless of the number of items equipped supplying that oskill: a Barbarian dual-wielding Call to Arms weapon is granted combined oskill bonuses of at most +3 to Battle Command, +3 to Battle Orders and +3 to Battle Cry. As mentioned above, +1 to All Skills from each weapon increases the level of these oskills.
Class-specific level bonuses to skill trees (skilltabs) and class skill levels can increase oskill level as well for that class (but not for a mercenary). The three aforementioned oskills granted to a Barbarian would thus benefit from bonuses to Warcries (Barbarian Only) as well as to Barbarian Skill Levels.
Equipping items with class-specific bonuses to an individual skill after an item with an oskill bonus to the same skill will result in the loss of that oskill, as will removing all items with the class-specific bonus after equipping the oskill item (either manually or by switching weapons in both cases). The oskill bonus can be restored by equipping the item supplying it again.
This does not apply to a Barbarian switching weapons while equipping a Passion weapon (+1 to Zeal) either made in a scepter with +(1-3) to Zeal (Paladin Only), or with such a scepter in his other hand.
When equipping an oskill item without any items with class-specific bonuses to the same skill, moving the cursor over an item which does supply a class-specific bonus to the same skill will also result in the loss of that oskill. Once again the oskill bonus is restored by re-equipping the item supplying it.