The online dictionary definitions of the word "Oath" do no include the idea that a deity is required.
1. a solemn promise, often invoking a divine witness, regarding one's future action or behaviour.
"they took an oath of allegiance to the king"
noun [ C ] UK /əʊθ/ US /oʊθ/ oath noun [ C ] (PROMISE) a promise, especially that you will tell the truth in a law court:
Medieval knights took an oath of allegiance/loyalty to their lord. The witness placed her hand on the Bible and took the oath (= promised to tell the truth).
However the UK Parliament Oaths act of 1978 makes a distinction between an Oath and an Affirmation for swearing in a member of parliament. Has the meaning of the word Oath changed over time? Is it common to use the word Oath in the meaning of a promise without requiring a divinity to witness said oath?