When you reach 3rd level, you swear the oath that binds you as a paladin forever. Up to this time you have been in a preparatory stage, committed to the path but not yet sworn to it. Now you choose an oath, such as the Oath of Devotion.
Your choice grants you features at 3rd level and again at 7th, 15th, and 20th level. Those features include oath spells and the Channel Divinity feature.