If you wish to imagine your Paladin serves some deity or gets their oath from some supernatural being there is nothing in the game that's stopping you.
Nothing is limiting your imagination.
If for some reason you have to have it on your character sheet you could always type it in your name field. For test purposes I just created a character named "Tav, servant of Helm"(You only have 20 characters so choose wisely)
For me it is the dialogue tags. If I am a paladin that is important to the NPC I am talking to I want to be able to bring that up to them like clerics of Shar or Selune can with Shadowheart
