khwan said:
Confirmed Okiura.
https://www18.atwiki.jp/sakuga/pages/8.html

Btw, I don't think Kou worked on the film.
Thanks, glad I could get this confirmed. I don't really look at the Japanese sakuga wiki and other sites like that but the flashback sequence of Mitsuha's past sure shares striking similarities to Kou's unique post-processing.