OAuth2, SAML, OpenID connect – Which one to use for my scenario?

I work for a company where we give customer (hundreds/thousands of users) access to 2 sites. One owned by a 3rd party SaaS and one owned by us. Customers spend alot of time registering for both sites and we also spend alot of time removing accounts when customers no longer need access.

I would like users to register for Site A. After successful authentication; a user can click on a link within the site to access Site B but without the user entering credentials. I want Site A identity to be used to access site B and its resources. I do not need site B resources to be presented on Site A site, but simply allow users to access site B if already authenticated to site A.

Users may have different roles on site B.

What is my best option? Oauth2 sounds like a good option. But will it satisfy my requirement above?

Who will manage the authorisation server? I presume Site B?

Thank you.