Steam OS is linux, Linux game list will pretty much work with Steam OS, so thats about 900 games, including Atilla Total War and soon to be Saints Row 4, Shadow of Mordor etc. Now that they are real games are being updated to support steam OS. From what I remember the number is close to 950 games with about 260 that could possible be ported short term.
yea bout 950 and 260 possible additions
https://steamdb.info/linux/
List of games on sale many of which may soon be ported to Steam OS. including
- Shadow of Mordor
- Payday 2
- Batman: Arkham Knight - Going to be a same-day release, currently pre-order!
- Saints Row IV
- Company of Heroes 2
- Magicka 2
- Total War: Rome II
- The Witcher III
- GRID Autosport
- Total War: Attila
http://store.steampowered.com/sale/steamos_sale