The barrow knights placed their oaths before their ancestors as they kept vigil for a night above their barrows. The last kind was a new thing, one of the many changes after the Sundering - the emerging winter knights, also called Thegns by the smallfolk, that gave their vows to the Heart Trees in the eyes of the Old Gods. There was some difference in the knightly orders - but the core remained the same - fealty for one's liege, to serve in defence of your people and the such, although there was quite some variation in the wording of the vows.