It wasn't fully open source, but mostly. It was merged with Intel's Moblin into MeeGo. Once Nokia stopped working on that, MeeGo was forked into the fully open source Mer. Mer is nowadays used by Jolla as a basis for Sailfish OS (which again has non-open source parts at the top of the stack).
https://en.wikipedia.org/wiki/Moblin
https://en.wikipedia.org/wiki/MeeGo
https://en.wikipedia.org/wiki/Mer_(software_distribution)
https://en.wikipedia.org/wiki/Sailfish_OS