Sorry. I overlooked it. Let me get you some answers.
Update:
I checked in 2.2.0b1 and OAuth2 support is not added to check_mail_loop yet. On check_mailboxes and check_mail it looks like the following at the moment:
--fetch-server ADDRESS
Host address of the IMAP/POP3/EWS server hosting your mailbox
--fetch-username USER
Username to use for IMAP/POP3/EWS
--fetch-email-address EMAIL-ADDRESS
Email address (default: same as username, only effects EWS protocol)
--fetch-password PASSWORD
Password to use for IMAP/POP3/EWS
--fetch-client-id CLIENT_ID
OAuth2 ClientID for EWS
--fetch-client-secret CLIENT_SECRET
OAuth2 ClientSecret for EWS
--fetch-tenant-id TENANT_ID
OAuth2 TenantID for EWS
Will check internally on this further and keep you posted.