This Gist provides examples of providing global IPv6 addresses inside OpenVPN tunnel.
If you have /48 subnet which is most likely routed prefix, see https://community.openvpn.net/openvpn/wiki/IPv6 If you have /64 subnet which is most likely link prefix, see openvpn-link64-v6-over-v4.md file If you have some other configuration (i.e. just /124 -- digitalocean, i'm looking at you), or just want to use weirdest config ever, see openvpn-nat6.md (which is yet to be created)
All configurations I've tested work great on Windows, Linux, iOS and Android. For macOS, either use Viscosity (paid), or see Tunnelblick/Tunnelblick#452 for Tunnelblick.