This spell line does have some select uses. Particularly before you get your Protection of the Cabbage/Glades type skin. IMHO, it is a good way to recover a little faster after a REZ out in the wild due to its low mana cost. Others feel it is useful in a situation where you are parked at a "camp" and someone else is pulling mobs to that location (outdoors, of course).
There is a catch, however...
Some years ago, the various treeforms were blocked in nearly every zone I tried This seems to have changed recently, at least a bit. If you think you have a use for this, try Treeform (Lvl.6) or Spirit of Ash (Lvl. 36) in the zone(s) you wish to use Spirit of Oak. If the lower level versions work (ie. cheap vendor purchase) you may want to seek out this spell.
Wish I had more time to try out treeforms in various outdoor zones and give a full report.