According to the docs (https://pantheon.io/docs/modules-known-issues#simple-oauth--oauth-20), because it is not possible to change file permissions on Pantheon, even in the private directory, you can't use the Oauth module out of the box.
How to fix this
- Create the keys and put them in your Pantheon private files directory (for example,
/files/private/oauth/public.key
, needs permission 770) or commit them to git outside the web root (needs permission 644).