Can I use open source OpenVPN clients to connect to OpenVPN Access Server or CloudConnexa

Yes, it's a standardized protocol, meaning that pretty much any client that uses the OpenVPN binaries to establish the VPN connection can connect to OpenVPN Access Server and CloudConnexa. There are some rare cases where custom implementations might not work or have difficulties, but generally speaking any OpenVPN-based client with a somewhat recent version of OpenVPN can connect to the Access Server. This includes OpenVPN client programs such as for example Tunnelblick for macOS, OpenVPN GUI for Windows, and even most implementations included in routers available on the market.