No rule for this one
No ending or word group predicts the article of Ohr. Learn it as a pair and say it out loud: das Ohr.
das Ohr
It's das Ohr. Ohr is neuter, plural die Ohren.
Mein Ohr tut weh.
My ear hurts.
No ending or word group predicts the article of Ohr. Learn it as a pair and say it out loud: das Ohr.