Operating Systems (OSes) have historically been classified according to their isolation properties: monolithic OSes, microkernels, single-address-space OSes, or unikernels... Decades of experience in research and industry showed that there is no silver bullet and that different use-cases might demand different approaches to optimize safety and performance. What if we tried to design an operating system able to be easily reconfigured into any of these points in the OS design space? What if the OS could be a microkernel, a unikernel, or a monolithic OS, at will, and using a wide range of hardware- and software-backed isolation mechanisms? In this talk, we will present FlexOS, the result of our recent research work in trying to answer this question. FlexOS is an OS allowing users to easily specialize the safety and isolation strategy of an OS at compilation/deployment time, instead of design time. Depending on the configuration, the same FlexOS code can mimic a microkernel with multiple address-spaces, a single-address-space OS with Intel MPK compartments, or many other OS isolation approaches. We have implemented a prototype of FlexOS on top of Unikraft, a popular library OS framework.