Maelle Salmon explains how OAuth 2 works and also how you can use it in R packages:
When writing an R package wrapping an API using OAuth 2.0 you’ll need the user to grant access to an “app”, which will allow to create an access token and a refresh token. The access token will then often be passed to the API in a header when making requests, whilst the refresh token would be posted in a query string when the access token needs to be renewed.
Your problem is: how do I imitate a third-party app? Thankfully for you, in most cases the complexity can be handled by the httr package. For other cases, or if you want to e.g. only use curl, you will have to get creative.
Read on for more detail.