it does not have a particular translation in english. Most writers leave it in that from.iT IS AN INVOCATION TO LORD SHIVA. IT IS USED WHEN A PRAYER IS MADE TO SHIVA. it is a kind of opening words of reverence to lord shiva.

Agree with telef that the phrase does not have a meaning you can readily translate since it is basically a standard chant, an invocation of the God Shiva.
Nevertheless, here are the meanings of the individual words, if that helps.
Om refers to God. Namah is a verb meaning to pray, to salute, to greet. Shivay (pronounced Shiva ay) stands for Shiva. HTH

This is a commonly occuring phrase in religious contexts. Please note that the phrase supplied is spelled incorrectly and should actually be
'OM NAMAH SHIVAAYA'. Om is the primordial symbol of creation in Hinduism. Namah is a phrase that approximately stands for prostrating, or otherwise bowing down to the person in question. In this phrase, the person in question is Shiva, one of the Trinity of Hinduism, also sometimes known as the lord of destruction.

Agree with the other 3 - this is indeed a typically Indian spiritualistic term, and as such, does not readily lend itself to any literal translation. The closest you can come to are the above 3 alternatives that I have come up with, if translate you must. Hope it helps.