No rule for this one
No ending or word group predicts the article of Öl. Learn it as a pair and say it out loud: das Öl.
das Öl
It's das Öl. Öl is neuter, plural die Öle.
Ich koche mit Öl.
I cook with oil.
No ending or word group predicts the article of Öl. Learn it as a pair and say it out loud: das Öl.