"Oskill" Points are skill points that can work for any class. For example:
The Trang-Oul's Avatar Set gives +18 to Fireball and +13 to Fire Wall. If a Necromancer is wearing a Trang-Oul set, he'll get +18 fireball and any +X To All Skills equipment can further increase this. The same applies to all other non-sorceress class wearing a part of that set.
However, oskills work differently for classes that "own" that skill. If a Sorceress used this set she would get a boost in Fireball.
If a character gets an oskill bonus to a skill that class "owns," the bonus is restricted to a maximum of +3. If a Barbarian is dual-wielding Call to Arms runeword weapons, the +3 cap is applied to the total from both weapons. This is all intentional, in order to balance these items.