I am working to create a WP plugin, and I need to connect to an external service using OAuth. I wanted to know if wordpress already include a OAuth client API or if I should use some external library, or if you can point me to what is the best practice in that case. thanks
You need the WP REST API - OAuth 1.0a Server from here https://github.com/WP-API/OAuth1
Here a Tutorial to install OAuth for the WP API https://www.sitepoint.com/wp-api-and-oauth-using-wordpress-without-wordpress/