Best nmap can do is guess. The way it guesses is by the reply of the open ports and such. If there are not many open ports to none it wont tell you anything as far as os goes.
You can try nmap -sT -O IPADDR
Sometime the only clue you will have is what is on the machines Info Card and what you can scrounge in the various discussions on the box in question.
There are other ways to find what specific os is on a box… example IIS is windows. Now if you find the version of IIS you can look it up on MS website and see what version the os is. Use your imagination and dont get stuck on it.