Why does U follow a long O sound?

2 comments

1. Because the people who designed the [1946 spelling reform](https://en.wikipedia.org/wiki/Modern_kana_usage) decided to keep it

   おう and えい used to be pronounced as two separate vowels, and in fact they still are by some speakers, or even most speakers when overenunciating, especially for えい.

   As an aside, before the new orthography was implemented おう (and other o-moras + う) could have been any number of different variations in the old system, like あう, あふ, わう, or already おう. Other than おう and えい another remnant of the old orthography are the particles は/へ/を, which would’ve turned into わ/え/お if the reform was fully consistent

2. I think the ou is for words of Chinese origin. Old Japanese words get prolonged by another o (oo). Ooki vs dou