Configuring OpenVPN ipv6-over-ipv4 are exactly two lines in your server config: server-ipv6 your-ipv6-subnet-for-clients push "route-ipv6 subnet-you want-to-ro

Configuring OpenVPN ipv6-over-ipv4 are exactly two lines in your server config: server-ipv6 your-ipv6-subnet-for-clients push "route-ipv6 subnet-you want-to-route" (either your real subnet, or 2000::/3 for all traffic). You need the same for the IPv4 side of things too. Clients need no change.