Maybe this app? There is no official app for OGS but this one is the unofficial one known
Oki. Then this is not what an app is, just a direct link to the page?
I mean an app is some standalone piece of software, not just a link and OGS don’t manage any app itself (so no official one). The two famous ones have a thread here in the forum opened by their creators.
I hope someone using IOS may help you.