How to setup OpenVPN server without Client Cert and using local user authentication

As the title suggests, I'm trying to find a way to use client username/password authentication without client cert to connect to the OpenVPN server. Ideally id prefer not to use the PAM module but standard local Shadow method instead for centralised authentication with local operating system users (useradd etc..).

I presume if the later is not an option you would need to run some type of RADIUS server.

Thanks for the assistance