Under Network> VPN > OpenVPN > [Session Name] > Options you can set various options for the OpenVPN client. Usually you can leave the default settings as they are. If the server uses compression, enable Use LZO data compression.
When using a proxy, set Protocol used for communication to the host to tcp-client.
Under Network > VPN > OpenVPN > [Session Name] > TLS-Options you can set various TLS-related options. In particular, you can configure whether the remote peer certificate will be verified.
For details about these settings refer to the OpenVPN howto or manual page (see Further Information)