I spesifically went for Oathbreaker on the one paladin I've tried and it wasn't too hard to achieve/understand. Basically I promised to help a couple people kill a third imprisoned person and upon releasing them I immediately turned on the first two, thus breaking the promise or "oath". As I recall from the patch 9 stream the "nature" paladins easily fall if they consciously commit violence or cruel acts upon animals/nature. So it seems the oaths mainly seem to break when you act upon your words, not when you merely say them. I.e. lying in my example didn't trigger oathbreaking, but acting on it did.
