See the following two images:
http://imgur.com/a/mkPFS
In the first image, you can see Xcode - a app made by Apple. It reports my 4S has 14.59GB.
In the second image, you can see iOS settings screen. It reports my 4S has 13.6GB.
Now, there are 3 numbers..
Number 1: 16GB - the number on the packaging, 1000 mb = 1 gb
Number 2: 14.59GB - xcode, this uses 1024 mb = 1gb
Number 3: 13.6GB - iOS, ???
As you can see, iOS clearly is lying about how much data you can store.
OS takes zero space, along with bootloader, whatever applications you have, user space, etc.