Sorry for the trouble this is causing!
OS X is specifically written to prevent a third-party application from crashing the OS; if that's what you're seeing, it means that something inside the OS is broken and is crashing the Mac, taking OmniFocus with it.
Most often, this is a 'reinstall the OS' situation, but if you contact our
support folks, we'll be happy to investigate this with you in case there's something else going on.