I'd say some parts should work out of the box (there many things that are completely OS agnostic, like containers, cache, bindings to other libraries like PCRE, etc.), and some other it would be quite some work (for example all the I/O is tied to epoll, there is one class to work with direct I/O that's super Linux specific, etc.).
Hm. I'd have to see what works. I can't afford to be stuck to one OS.