I saw page when I was Searching with the key word “Open source”
http://www.opensource.apple.com/release/mac-os-x-1064/
I just want to ask whether it is possible to build a entire OS using the source code ?
and if yes then what exactly needs to be done.
Yes it is possible. That is exactly what Apple does to build its Mac OS X. However, Mac OS X is not open source so you can not get its source code in order to build it. Apple releases only some parts of their OS and what you can build from it is called Darwin.