Yoku Wakaru Mahouka!

Both are short ONAs made as a spin-off of otherwise action series. Both are made in kinda of chibi style and both are comedies. Both have mainly characters from the anime.  

Sure, Wake Up Girls and Mahouka Koukou no Rettousei are completely different in genre, but these two ONAs have their share of similarities. For example, they both have a strikingly similar animation style; all characters involved are chibi versions of their original counterparts. Additionally, the ONAs serve a different purpose than their series of origin (WUG Zoo is comedy-based rather than music-based; Yoku Wakaru Mahouka focuses on educating the viewers on the lore of Mahouka Koukou no Rettousei rather than narrating the characters' experiences in their world). So, these ONAs are recommended as a way to bring levity to their otherwise serious anime series of origin through the use of chibi art and an extra dose of comedic effect. read more 

Both are short ONAs made as a spin-off of otherwise action series. Both are made in chibi style and both are comedies. Both have mainly characters from parent show talking with other characters. 

Cute chibi character that explains the events of the parent anime that are difficult to understand. Fate/Zero and Mahouka Koukou no Rettousei are highly recommended! 

Chibi style characters from the main series. Some episodes pertain to the current parent episodes. While others give you info about the world of that series. In Mahouka's case, we learned about the school life, magic attributes and other things.