The Shiva Purana says:
"The chanting of the five-syllabled mantra shall always be performed along with Om" (Shiva Purana Vidyeshvarasamhita:17.34).
Thus the potent mantra becomes: 'Om Namoh Shivai.'
It is further stated:
This mantra is of the nature of Shiva himself. By holding on to the mantra, the physical body of the devotee becomes identified with Shiva. (Shiva Purana Vidyeshvarasamhita:17.132).