Whether the server is free / open source software is irrelevant to the matter of privacy with a VPN. It's running on someone else's computer, so you have no way of proving what is running, or more importantly, what
isn't running - the service provider can run OpenVPN and also tcpdump, both of which are free software. You need to trust the provider not to monitor your traffic, and perhaps not to be easily compelled to monitor your traffic on someone else's behalf.
(The same is true of Tor exit nodes, incidentally, and it's very easy for an intelligence agency to run Tor and tcpdump.)
If you actually want a VPN, one of your best options is to use a commercial service that has a reputation to uphold. Some fly-by-night "non-profit" is probably a front for a miscreant running tcpdump. (And there is no conflict with a commercial service running open source code, as I'm sure you know!)