WAmigaOS
WAROS Research Operating System
WExokernel
WFreeRTOS
WGNU Hurd
WGreen Hills Software
WHelenOS
WMac OS 8
WMeiko Scientific
WMicroEmpix
WMicrokernel
WMINIX
WMINIX 3
WOS2000
WPhantom OS
WPikeOS
WPowerUP (accelerator)
WQNX
WRIOT (operating system)
WSharpOS
WSingularity (operating system)
WSymbian