Recently I was contacted by an Author of WP OAuth Server informing me that there was a demand for integration between WP OAuth (https://wordpress.org/plugins/wp-oauth/) and WP OAuth Server (https://wordpress.org/plugins/oauth2-provider/).
With no time wasted, I hopped on the matter and submitted a Pull Request at https://github.com/perrybutler/WP-OAuth. The PR provided configuration that allows for a user to connect WP OAuth to WP OAuth Server and allow users to login via another WordPress site using Single Sign On.
You can download WP OAuth at https://wordpress.org/plugins/wp-oauth/.
This is a free alternative to https://wp-oauth.com/downloads/single-sign-on-client-for-wordpress/. WP OAuth is not supported by me so if you have issues you will have to visit https://wordpress.org/support/plugin/wp-oauth.