Namaskar. My question is related to the mantra 'Om Namaḥ Śivāya'. Can a Mleccha/uninitiated person chant it fully or should he/she just chant 'Namaḥ Śivā'? Even without the 'Om' in the beginning, 'Namaḥ Śivāya' is still a Vedic mantra (Panchakshari mantra) and Mlecchas are not supposed to chant Vedas, so I'm really not sure what to do. And if this mantra is off limits for such people, is there any other mantra which is safe to chant for Mleccha/uninitiated person? Thank you.
First dealing with uninitiated persons.
The mantra you are talking about is a Vedic mantra. Can anyone chant it without initiation?
The answer is usually NO as per scriptures. Without Vedic initiation (Upanayana) one must not chant any Vedic mantras.
Manu Smriti 2.172. (He who has not been initiated) should not pronounce (any) Vedic text excepting (those required for) the performance of funeral rites, since he is on a level with a Sudra before his birth from the Veda.
The Tantras also have the same opinion about chanting mantras and the need of initiation.
AdikshitA ye kurvanti japa-pujAdikAh kriyAh |
Na phalanti priye teshAm shilAyAmupta-bijavat ||
Whatever acts the uninitiated does, whether it's Japa, PujA or anything else, don't bear any fruits just like seeds sowed on stones don't grow into plants.
KulArnava Tantram 14.96
Therefore, unless initiated one is not qualified to do mantra japa.
Now, coming to Mlecchas. Such a person will not obviously have Vedic initiation. But he can be initiated as per Tantras.
Shudra sankarajAti nAmAdi shuddhirvidhiyate |
PAdodak pradAnAdyaih kuryAt pApavimochanam ||
Primal purification is ordained for the Sudra and mixed-caste (Sankara-JAti); one is freed from sin by using the water with which the feet of the Guru are washed, by gifts etc.
This verse is saying that every person is eligible for initiation but before initiation a mixed caste or a Mleccha person will have to undergo extra purification rituals when compared to a twice-born.
And, as per Tantras, after initiation (Diksha Samskara) the caste system dissolves. And, one is qualified to chant any mantras after that.
Gatam Sudrasya sudratvam ViprashyApi cha VipratA|
DeekshasamskAraSampanne JAtibhedo Na Vidyate ||
After Deeksha Samskara a Sudra does not remain a Sudra, neither a Vipra remains a Vipra.There do not remain any differences of caste after Deeksha.
KulArnava Tantram 14.91
Yes, I think anyone can chant Om Namah Shivaya Mantra but with certain exception/changes.
In the chapter Glory of Pranava Panchakshara Mantra [Chapter 17 Videsvara Samhita Shiva Purana] various powers of Panchakshara Mantra are told. There Sage Suta also tells that Panchakshara Mantra even can convert Sudra into a Brahmana. As stated in chapter:
क्षत्रियः पञ्चलक्षण क्षत्रत्वमपनेष्यति । पुनश्च पञ्चलक्षण क्षत्रियो ब्राह्मणो भवेत् ।। मन्त्रसिद्धिर्जपाद्यैव क्रमान्मुक्तो भवेन्नर । वैश्यस्तु पञ्चलक्षण वैश्यत्वमपनेष्यति ।। पुनश्च पञ्चलक्षण मन्त्रक्षत्रिय उच्यते । पुनश्च पञ्चलक्षण क्षत्रत्वमपनेष्यति ।। पुनश्च पञ्चलक्षण मन्त्रब्राह्मण उच्यते । शूद्रश्चैव नमोऽन्तेन पञ्चविंशतिलक्षत ।। मन्त्रविप्रत्वमापद्य पश्चाच्छुद्रो भवेद्द्विज। नारीवाथ नरो वाथ ब्राह्मणो वान्य एव वा ।।
A Kshatriya should recite this mantra five lacs of times, then he is freed from duties of a Kshatriya. By again reciting the mantra by five lakhs (five hundred thousand) of times more, he becomes a Brahmana. By again reciting the same number of the mantra, one achieves the siddhi in the mantra and one gets free from reciting the japam. By performing the japam of the mantra five lacs of times, a Vaisya is freed from Vaisyatwa or remaining as Vaisya. By reciting the mantra for five lacs of times, it is called as mantra Kshatriya. By reciting further by five lacs of times his Ksatriyahood is removed. By further reciting the mantra by five lacks times more, he is called the mantra Brahmana. In case a Sudra adding Namah at the end of the mantra, recites the mantra by twenty five lacs of times, then he achieves the position of a mantra Brahmana. After reciting the japam by twenty five lacs of times again, a Sudra is turned into a pure Brahmana. Irrespective of the one, being a woman, man. a Brahmana or anyone else, all are purified with the reciting of this mantra.
Taken from https://hinduism.stackexchange.com/a/16418/17858 .