Om Namah Shivaya (Aum Namah Shivaya) is a Mantra highly prevalent in Hinduism. It is considered to be a mantra of Lord Shiva (Siva) the destroyer among the Trimurti's (Brahma, Vishnu, Shiva)
Shiv Puran (A Holy text of Hinduism) gives the description of the Panchakshara / Panchakshari mantra in great detail. In there it is described that the "Om"(ॐ) Mantra is made of five syllables and is in reality the actual "Panchakshara / Panchakshari" mantra. Shiv Puran describes that the "Om"(ॐ) Mantra is made of five sounds is thus the actual Panchakshara / Panchakshari Mantra whereas translators of Shiv Puran have adulterated the mantra while doing the translations and have added "Om Namah Shivaya" (ॐ नमः शिवाय) on their own. In reality "Om Namah Shivaya" (ॐ नमः शिवाय) is not the Panchakshara / Panchakshari Mantra and is indeed self made and arbitrary.