Inbound configuration
Roundup's roundup-mailgw has support for polling an IMAP server that support OAuth/OAuth2 (e.g. Google, Microsoft Office 365). For POP support with OAuth, using the email-oauth2-proxy below may work.
Outbound configuration
If you are trying to send email from Roundup through a provider that requires OAuth2, this Python email proxy may be useful: https://github.com/simonrob/email-oauth2-proxy.
You can configure Roundup to the running proxy as your SMTP server by setting host=127.0.0.1, with the username set to the value that identifies your email account configured in email-oauth2-proxy. Also configure the port to the port where the proxy is listening.
This should work, but has not been tested yet. If you use this please report your success/failure on the roundup-user mailing list.