Uncle
noun
An uncle on your father's side is still your uncle. This uncle always has to be younger than him though.
おじ
The reading for this word is an exception. But you already learned the 伯父 which means uncle too. Well they have the same reading! So if you know one uncle, you know them all!
叔父はこの学校の校長です。
My uncle is the principle of this school.