From the [[Wurm_Server_Release_Notes_2011#110715_Fo_priest_buff.Horses_and_guards_toned_down|July 15, 2011 update]]:
* Oakshell power now uses the rule of 70 algorithm and may glance
* If the power of an Oakshell is above 70 it will grow into a Thornshell and cause wounds like Area [[Aura of Shared Pain ]] where you wear no armour. The effect name won't change yet though.
As of the [[Wurm_Server_Release_Notes_2012#120215_Movement_changes|February 15, 2012 update]], armor while worn will make Oakshell inactive on that bodypart.