Oak Sage is a powerful summon in the Druid's arsenal. What it does is simply to raise the life of both the druid and any minions or party members with him, but the bonus can be staggering and a truly helpful aid, especially in later nightmare and Hell where enemies start to deal large amounts of damage.
The good thing about Oak Sage is that virtually every class and build out there benefits from it. Melee characters such as Barbarians and Paladins always need more health, and weaker classes like a Sorceress benefit as well. Instead of dying from a stray attack (by Gloams for example) the Sorceress can survive due to the health bonus.
The druid can however only summon one spirit at a time, and it often becomes choice of choosing between Oak Sage and Heart of Wolverine. The latter provides damage and attack bonuses to the druid, and invaluable help for a melee-oriented shapeshifter. Elemental and Summoning druids however most often benefit from Oak Sage much more.