Since the description given in game can be summed up as...it is bad, don't do it.
I will list exactly what breaking the oath does. I have tried all this stuff.
Subject to change.
You break the oath by using a forbidden type of armor or a non- wood shield.
Note: Wooden Tower shields are fine...if you know how to use them.
Weapons have nothing to do with oath breaking..if you got prof. ..go for it.
Wearing a runearm also breaks the oath.
OK...so what happens?
You cannot train up a druid level if your currently BO - Breaking Oath
You cannot change into animal form
You cannot throw a druid spell from memory
OK...what can you still do?
You can call your pet - not the spell summon animal
You can use that ranger like animal placitate skill.
Any druid spell that you cast before you were BO, stays active til it runs out. So, you can buff
...with your armor off then put it on...buffs stay full duration.
Druid enhancements that are not based on animal form keep working. So if you
...get 2DR for 6 seconds after you intimidate...it still works even in BO status.
Untested
flameblades in BO status
the rotating winter/summer enhancement thing