The Oxwall server requirements are for both Windows and Linux.
If you
google which OS is the best for hosting then you find tons of discussions about this topic, but mostly Windows is advised for web applications/database developed in ASP/ColdFusion in combination with MSSQL.
Because Oxwall is developed in PHP/MySQL/Java you have a free choice to use any OS which supports these configurations. Dozens of people (including me) see the advantages of using Linux instead of windows, but it's up to you to decide which one it is gonna be..