How do we OR two bits in ATxmega128?
For OR you use the pipe symbol:

|

This is a vertical bar, usually the symbol is close to the enter key on the keyboard;

For example:  int x = BIT0 | BIT 1;

However, seeing as you're asking rather basic questions, I recommend that you find a tutorial or book to cover this. K&amp;R which is the C book covers this. I myself have written a tutorial (aimed at MSP430 but covering many general aspects) here.