There is this nice overview: https://wiki.openwrt.org/doc/howto/vpn.overview
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https://wiki.openwrt.org/doc/howto/vpn.openvpn
https://wiki.openwrt.org/inbox/vpn.howto
https://wiki.openwrt.org/doc/howto/openvpn-streamlined-server-setup (there is mentioning setup to have openVPN as ‘proxy’ for all traffic )
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might by handy as well https://wiki.openwrt.org/doc/howto/vpn.client.openvpn.tun
and sure you will need https://openvpn.net/index.php/open-source/documentation/howto.html
or https://community.openvpn.net/openvpn/wiki/HOWTO
and for fine tuning you configs https://community.openvpn.net/openvpn/wiki/Openvpn23ManPage
As there are different examples/goals in each guide(and some are out-dated or not fully applies for TurrisOS), sometimes it is confusing what is necessary to make as next step. Especially when you need something special and not just p2p with static key or bridged-tap.