We have SP Farm having 1 Front End & 1 Web App Server. Current OS is Win Server 2008.

Question:

Can we add two more servers in farm having (OS Win Server 2012), after adding servers can we move Central admin and all other stuff to Win Server 2012 and later decommission Server having Win Server 2008.

Idea is to upgrade the OS from 2008 to 2012.

The platform must be the same across all related servers, so SharePoint servers on same matched OS and service pack versions, SQL on it's matched OS and service pack versions. So no, you cannot add 2012 app or WFE servers to a farm with an OS running Windows 2008.

What you can do though is build new win 2012 servers and do a pysical to virtual capture of the win2008 servers and run the farm virtualised on a single server if your capacity demnds permit this.

This would at least allow you to start building a replacement farm on a base 2012 OS utilising that existing server infratructure and then migrate the content databases into that new farm, treating it as an upgrade project in effect.