OpenVPN is a popular open source VPN protocol and it is cross platform. While OpenVPN runs on OS X there are no official packages for it. Most VPN providers offer their own OS X binaries that will usually support OpenVPN. A popular proprietary cross platform OpenVPN client is Viscosity. It is reasonably priced at $9 and there is a 30 day trial for Windows and OS X. But there are many reasons beyond cost, why a user may want to use an open source client.
A popular open source OpenVPN client for OS X is Tunnelblick. Tunnelblick is released under the GNU General Public License (GPL) and as such all the source code is available. I went ahead and downloaded the latest source repository to take a look. Technically, Tunnelblick is a Graphical User Interface (GUI) that interfaces with the OpenVPN code. The repository includes the OpenVPN source code as well as a few other open source projects it makes use of such as the popular OpenSSL project.