When we learn the vowels in western countries, we always follow in the order aeiou.
But chinese children learn them in a different order aoeiu.
This has some importance, because this order is used to define which vowel is stressed when there are several consecutive vowels in pinyin. The most stressed vowel ( a first then o …) has the pinyin diacritic , except if there are only u and i the diacritic is always placed on the last one (liù duì).
Let see this episode of Say It Right serie 
https://chinesepod.com/playlist/say-it-right/pronunciation-9