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 (...
Why is it required for it to be actually effective for the reciter?
The reason is - "the scriptures say so". That Mantra Japa is fruitful only if the Mantra is Guru-given and otherwise it is not.
For example, Lord Shiva says:
PAramparyAgamAmnAyam mantra AchArAdikam priye | Sarvam
gurumukhAllabdham saphalam syAnna chAnyathA ||
Om is a Mantra, so chanting Om constitutes Mantra Japa. Other names for it are Pranava Mantra, TAraka Mantra etc.
Om represents the Adi Dvani (primordial sound) and is an Ekakshara Bija Mantra (mono-syllabled seed Mantra). It also represents the Brahman in the MAntrik level.
EkAkshare tathA kute traipure mantra nAyike | Shtri datte swapna
lavdhe cha ...
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:
The word mantra in Sanskrit means ‘that which protects by being meditated upon’ (mananAt trAyate iti mantrah).
Mantra can contain bijAksharAs and name of the deity.
Example: chAmundA mantra ॐ ऐं ह्रीं क्लीं चामुण्डायै विच्चे।
Ganapati mantra : ॐ Gam Ganapataye Namaha
or may contain words like gAyatri mantra:
ॐ भूर् भुवः स्वः ।
From Om Viṣṇupada A.C. Bhaktivedanta Swami:
According to the rules and regulations of fruitive activities, there is a need to offer periodical food and water to the forefathers of the family. This offering is performed by worship of Viṣṇu, because eating the remnants of food offered to Viṣṇu can deliver one from all kinds of sinful actions. Sometimes the ...
The full process of Japa in short has the following steps:
Taking the Sankalpa (stating the purpose of Japa and revealing to the World that you will be doing the Japa)
Performing Rishi-Adi Nyasas ( 6 limbs of a mantra to be planted at their respective places on our body)
Doing Kara Nyasa and Anga Nyasa (for invoking divinity in our own body and for making ...