Oaths are rituals of binding using both Spirit and Divine magic. As long as the sworn tenants stand true, the one who swears by them can expect the powers promised to stand true. That said, if a tenant is broken, the Oathbreaker (DMG subclass) is infused purely with the magic of the spirits of those who have broken their Oaths in the past and are haunted by them.
The Ritual of Swearing Oaths
Paladins get their power form the magic of their Oath, while other campaign settings treat Oaths as more religious in nature. Here on Tor'Dath Oath's are rooted in ritual enchantments.
The origin of the creation of the Oath rituals is largely unknown, but often The Church of the Sacred Flame is followed by many paladin orders.