From my understanding:
If you did so intentionally and the crime is severe enough, it use to be that the god would curse you several times over and turn you into a Death Knight(powerful undead) and be labelled an Oath Breaker. Doomed to seek redemption for as long as the god says so. When or if you redeem yourself the god would claim your soul and you'd be welcome to their realm.
Nowadays you just lose all power and either you suck at life or another god picks you up. Oath Breaker devolved to be an evil paladin that simply serves something evil.
As far as I know Oathbreaker is still that, where the Paladin in question has broken their vows completely and broken off from their god without following a new one or has acted completely antithetical to the gods? Evil paladins still serve a god, albeit an evil one, so they still act as a paladin like Paladins of Lolth or Shar and are thus not an Oathbreaker, I think?
