Head on over to the Ray Wenderlich tutorial site to view my latest tutorial, How to make a game like Candy Crush tutorial: OS X port. This tutorial explains how to take an existing iOS-only Sprite Kit based game and turn it into a cross-platform iOS and OS X game.
While the Sprite Kit framework itself is the same across both iOS and OS X, differences arise when you see that on iOS it inherits from UIKit, and on OS X it is built on top of AppKit. This has implications especially for things like event handling (and, of course, app bootstrapping), so inevitably you need to write some platform-specific code, but it is not that hard to minimise.
Go read the tutorial for all the details!