Oak healing is like this:
Say you went out with your HoW, and you get damaged, cast Oak and wait til the aura thingy is under you. Most people who use Oak healing has Oak as their main spirit tho. (Or they might have maxed/stacked both.
If you went out with Oak, you got damaged, you'd either unsummon or cast another spirit, so the Oak aura isn't on you. Then recast Oak Spirit.