To connect using OpenVPN in the client, click on the Cogwheel icon and then the Connection icon.
You'll be presented with a bunch of settings for the VPN connection, toggle the option Connect using OpenVPN to Enabled to activate OpenVPN in the client.
With OpenVPN enabled, you'll connect using the OpenVPN protocol instead of WireGuard.
OpenVPN is an older and more established protocol than WireGuard that offers slightly higher privacy than WireGuard due to its dynamic IP addresses and use of username and password instead of keypairs.