Install OpenVPN with a valid web server SSL certificate (e.g. PositiveSSL, RapidSSL) - Installation, Configuration
OpenVPN documentation features a good tutorial on this topic, which can be found here:
https://docs.openvpn.net/configuration/installing-a-valid-ssl-web-certificate-in-access-server/
https://docs.openvpn.net/configuration/installing-a-valid-ssl-web-certificate-in-access-server/