The Documentation is not quite clear to me.
It states:
"For now the API only supports two authentication schemes: session based authentication and basic auth."
And the it mentions OAuth2.0 authentication.

So, is it possible (e.g. for an iOS app) to use oauth2 authentication?

The first paragraph is indeed outdated. OAuth2 is indeed possible with Authorization code flow and Client-credentials. You can register a custom application in your OpenProject instance at Administration > Authentication > OAuth applications

I fixed the paragraph in https://github.com/opf/openproject/commit/67c0eb111e

Cheers

Oliver