It’s very easy in OSMC - and we do use systemd.
To enable OpenVPN to run at startup, run:
sudo systemctl enable openvpn
To specify which config file to run, you need to:
1 Place the config file in /etc/openvpn and give it a .conf suffix, not .ovpn.
2 Edit file /etc/default/openvpn and add a line
AUTOSTART=“NL”
so that it will look for file /etc/openvpn/NL.conf.
Then restart and see if it’s working as you expect.