Well 'okaasan' is the term for 'mom' in general, and 'haha' is like a humble reference term. 'okaasan' is probably the most common way of calling your own mom like you call your mom 'mom'.
You don't directly call your mom 'haha' but only use 'haha' when you refer to your mom in a conversation with someone you're not that close to or older like your teacher or sempai (times when you should be humble). So with friends you use 'okaasan' when you talk about your mom.
Hope this answers your question a bit.