For Windows OS the recommended OS is Windows Server 2003 R2 (minimum requirement) or Windows Server 2008 R1 and R2.
Other OS for WebOffice Application Server are possible, if they meet the system requirements of the other software components. Please contact SynerGIS Support for this: web-support@mysynergis.com.
Note that installation of Esri ArcGIS Server is supported on Windows Operating systems (Windows 2003 or 2008 (including R2) Server) only.
Note: Using the 64bit versions of Windows OS on application server allows the usage of Java in 64bit mode and Apache Tomcat 64bit. This has the advantage of no limitation of the maximum memory pool of java processes will be needed.