I don't know precisely how strict the system will be or if every evil conversation/action is properly flagged by the game to break your oath but you will for sure be more restricted in conversation and you'll have to be very mindful of what you do. Like when Minthara asks you to infiltrate the druid camp and help her kill everyone there. You can probably agree to the plan without becoming an oathbreaker because you can still turn on her and sort of lead her into an ambush but if you actually decide to commit to helping her and destroy everyone there then you will be an oathbreaker. But sometimes a conversation choice leads to an immediate evil action that you commit in a cutscene so there's no taking back those. So if you really care about maintaining you starting Paladin class you do have to mind what you say and what you do.
