Is Overleaf open source?

Update 20th July 2017: We're delighted to now be working with the open source LaTeX editor, ShareLaTeX (read the full announcement here). Both Overleaf and ShareLaTeX are committed to ensuring that all of the open ShareLaTeX code base will remain open source and will continue to be actively developed.

You can access the ShareLaTeX repository on Github at https://github.com/sharelatex/sharelatex