I would like to add also, that there are a bunch of projects out now that implement Oauth2/OpenID for self-hosted identity providers (ory.sh / keycloak / nextcloud/gitea/gitlab/…).
This makes it possible to secure your services with a proper newer workflow to handle access of resources.
A solution to add first Google only or a static list is fine for me, only wanted to mention, that there is also need for a decentralizes solution.
I think currently there is no RFC how to get information about the responsible oauth provider in caldav/carddav, probably worth to think about a standard.
With google you have predefined information https://accounts.google.com/.well-known/openid-configuration
But there you need to registrar an app, before it can use the API, what is than shipped statically with the app, and not works for decentralized systems.
A solution would be a .well-known url for where to go to get a access token with a predefined API, this would make it flexibel for different use cases, not sure if this in the thinking of OAuth2 standard.
Normally OAuth2 wants a separate client_id for each “App” (Webservice, Phoneapp, Desktopapp, …), there is the workflow for a Mobile App what, is more or less, what google recommends.
for reference the corresponding RFC:
A solution would be “dynamic client registration management” mention here: https://tools.ietf.org/html/rfc7592
To create a new client_id on demand at the oauth2 server.
I started research not so long ago,
probably I can give you in the future some more feedback.
Implementation in the APP:
The implementation itself should be not so much, use a lib for OAuth2 add configs for provider (client_id,client_secret,authorization_endpoint,token_endpoint) create an request,
redirect use to a browser, a bit back and fore, in the and you get a access token, with what you can access the resources by adding the header Authorization: Bearer <token>. Normally the OAuth2 library does that for you, in this case need probably other adjustments in the app in the network API layer.
If you have any question I open yo help, also to test something except google services 
For the test server I currently wait until some other project have more progress ( https://www.ory.sh/oathkeeper ).
Otherwise I could setup a quick and dirty setup for testing if needed.
