The title of this post took me about 3 attempts to parse, because "OS" is more strongly bound to Operating System in this context (but presumably it's supposed to read as Open Source)
- FOSS: Free / open source software
- OSS: Open source software.
Edit: Did a quick google search and it seems like the acronym OSS has fallen out of prominence. Probably best to explicitly state “Open Source” nowadays to avoid any confusion.