Uncle
noun
An uncle feels like a father sometimes, but he's still an uncle after all.
おじ
You already know the お reading for 叔, but じ for 父 is new, so here's a mnemonic to help you: One thing nobody knows is that your uncle is none other than Jesus (じ)! That technically makes God your grandfather. Don't tell anybody about this, though.
叔父はこの学校の校長です。
My uncle is the principal of this school.