Using 'oaf' implies a strong negative opinion about someone's behavior or intelligence. He was being an oaf, disrupting the whole class with his loud behavior.
Pro Tip 2/3
Informal Usage
'Oaf' is informal and could be rude if used in polite company or formal situations. I whispered to my friend that the waiter was being an oaf for dropping the dishes.
Pro Tip 3/3
Personal Insult
Calling someone an 'oaf' directly can be very offensive; it's best used humorously or among friends. She laughed and called me a clumsy oaf when I tripped over my own feet.