I'll point you to answers to this question: https://www.odoo.com/forum/help-1/question/beginner-questions-deploy-openerp-odoo-in-linux-or-windows-servers-56408.

Traditionally Windows may have issues related to POSIX compliance that makes cross-platform supports difficult.  Remember that Odoo is build using Python and some support packages written in Python may not be available in Windows.  AFAIK, all support packages used by Odoo are available in most OS but if you use 3rd party modules, they may require support packages that are not available in Windows (there are some support packages that is available in Windows only as well, but it is rarer).

Another gotchas for Windows is the fear of malware attack.  It is true that Windows are traditionally the "go-to" OS for malwares due to it's popularity, however with popularity growing for Linux based system (especially Android), the number of malwares attacking Linux system also have grown.  The same goes to OS X.

So, looking into the facts, for now, it seems Linux based OS still have the upper hand as the "go-to" OS platform. Now, Linux has also come in myriads of flavour, so do choose a distribution that is more popular.  Also, version can sometimes be a tricky choice as well, like Odoo's Report Designer does not run on top of Ubuntu 14.04 out of the box because OpenOffice dropped support for Python 2.7 starting from v4 (the one used in Ubuntu 14.04) without a major effort.

Disclaimer: I personally have seen Odoo running on Ubuntu 12.04 and CentOS 5.10 quite OK.