Most modern DSLRs do not run consumer operating systems like Windows or Android. They typically use proprietary embedded firmware.

For Canon, older DIGIC II and DIGIC III cameras are reported to have used VxWorks, while DIGIC 4 and later cameras are generally associated with Canon’s DRYOS, which is often described as being built around or derived from µITRON/ulTRON concepts. Projects like Magic Lantern and CHDK have helped reveal some of this.

For Nikon, current DSLR firmware is generally considered proprietary as well; the provided answers do not identify a public OS name for Nikon DSLRs.

For other brands, some Sony cameras have been reported by firmware-hacking projects to use Linux components such as BusyBox. A few cameras have used Android—for example, the Nikon Coolpix S800c—but that is not typical for DSLRs.

So the short answer is: Canon and Nikon DSLRs mostly run proprietary embedded operating systems/firmware, not general-purpose desktop or phone OSes.