Blog post: tylergaw.com/articles/netlify-cms-custom-oath-provider
A working example of a custom GitHub OAuth Provider for NetlifyCMS.
This is the client side. Where you log in to the NetlifyCMS.
The server side is available on Glitch: https://glitch.com/edit/#!/netlify-cms-github-oauth-provider-example
Most of this example follows the standard NetlifyCMS setup. A key to making this work is adding the base_url setting in config.yml.
That tells NetlifyCMS where to look for the OAuth server.
backend:
name: github
branch: master
repo: tylergaw/netlify-cms-github-oauth-provider-client-example # change this to your repo
base_url: https://netlify-cms-github-oauth-provider-example.glitch.me # change this to your OAuth server