Running applications for one operating system (OS) on another OS is useful in many cases. However, porting applications requires high development costs and running applications in a virtual machine poses resource sharing problems. A promising approach is to use an OS compatibility layer that converts the guest application’s interface into the host one. Unfortunately, existing OS compatibility layers sacrifice either robustness or flexibility due to in-kernel subsystems or user-space-only implementations. This paper proposes a new architecture of OS compatibility layers that achieves robustness with almost user-level implementations while improving flexibility by exploiting standardized virtualization interfaces supported by most modern OSs. Our implementation of a Linux compatibility layer for macOS called Noah can run many of Ubuntu Linux binary applications, and a prototype implementation of a Linux compatibility layer for Windows confirmed the generality of our approach. Our experimental results demonstrated that the overhead of Linux kernel build time on Noah was 16%.