What would your goal be? I think locking a modern, general-purpose OS to a small number of cores would artificially constrain performance, assuming a reasonable scheduler.
Scheduler wise it would become easier as no juggling cycles upon main core that the OS is running upon. Whilst not hitting real-time OS levels, it would be a nice middle ground in some area's as well.
Also for cores, the OS and user software don't need the same OS when the OS is just API calls passing parameters, so would be viable to have the OS upon one core type and the user-space cores a totally different architecture. That again would add another level of security.