Is OpenVPN interoperable with L2TP?

Answer: No, L2TP is an incompatible protocol.

Details: L2TP, the Layer-2 Transport Protocol, is similar in some ways to the older and deprecated PPTP (Point-to-Point Tunneling Protocol) in that often the packet payload is a PPP (Point-to-Point Protocol) data link.  It's also similar to OpenVPN in that the transport layer is in UDP packets.

Since L2TP alone provides no form of encryption and confidentiality, it is often combined with IPsec to provide a VPN-like connection between endpoints.

OpenVPN is an SSL (Secure Sockets Layer) VPN, which uses the same encryption software and protocols as do HTTPS web servers.  It's now more accurately called Transport Layer Security or TLS, which superseded SSL.

Why SSL VPN?
What is an SSL VPN?
Three Benefits of Using an SSL VPN for Business

If you have additional questions, please submit a ticket.