(65) Song of the Oak should provide its usual buffs when used, slightly increased buffs while in sunlight, however one element of the buffs - The temporary HP - seems to be being granted to hostiles in the areas of effect as well as teammates. As far as I can tell, no other buffs from the bard song (+ab, + positive damage, regen etc) are provided to NPC's.
The temporary HP should onl;y be granted while in sunlight, I've not had opportunity to test if NPC's are getting it in darkness as well.
That is correct!