It is possible to use both SAML and OAuth plugin. You might need to combine both the plugins in the following scenario:
Your Identities are stored on two servers, one of which supports SAML while the other supports OAuth
In this case, you can configure the IDPs in their respective plugins and change the name of buttons in both the plugins so that it’s easy for them to understand which one to use. In case of multiple SAML IDPs, a dropdown is shown instead of buttons which display the list of IDP names you configure in SAML plugin.