Disclaimer: Currently I am employed by Microsoft, but my views and thoughts are still my own. The reason I joined Microsoft was, the work Microsoft have been doing for last couple of years in Open Source Space. Today I am a advocate for Open Source representing Microsoft.
To some people, OSS stands for Open Source Stack, others think of OSS as Open Source Software and now at Microsoft we would like to think of OSS as Open Sources Services. But to be honest for me OSS stands for all three.
You might ask yourself, now I know what OSS is - but what is Open Source?
Open source is a term denoting that a product includes permission to use its source code, design documents, or content. It most commonly refers to the open-source model, in which open-source software or other products are released under an open-source license as part of the open-source-software movement. Use of the term originated with software, but has expanded beyond the software sector to cover other open content and forms of open collaboration.
In other words (in my opinion), the source of something is open for either taking inspiration from or for building upon, but it is not necessarily free.