http://www.techradar.com/news/learn-how-your-iphone-works-with-apples-newly-released-source-kernel

Apple has released the source code kernel that its mobile operating system iOS is based on. Now, this doesn’t mean that you’re going to be able to easily build your own operating system off the back of it, but it will be interesting for developers to see how Apple handles basic tasks on iOS. 

The kernel doesn’t handle any of the higher level functioning of the operating system like apps, interface, or developer frameworks. That said, if you wanted to build your own operating system off the back of it, it is theoretically possible.