OM is the mantra, the expressive sound-symbol of the BrahmanConsciousness in its four domains from the Turiya to the external or material plane. The function of a mantra is to create vibrations in the inner consciousness that will prepare it for the realisation of what the mantra symbolises and is supposed indeed to carry within itself. The mantra OM should therefore lead towards the opening of the consciousness to the sight and feeling of the One Consciousness in all material things, in the inner being and in the supraphysical worlds, in the causal plane above now superconscient to us and, finally, the supreme liberated transcendence above all cosmic existence.
One must say, "Since I want only the Divine, my success is sure, I have only to walk forward in all confidence and His own Hand will be there secretly leading me to Him by His own way and at His own time." That is what you must keep as your constant mantra.
In this yoga there is no fixed mantra, no stress is laid on mantras, although sadhaks can use one if they find it helpful or so long as they find it helpful. The stress is rather on an aspiration in the consciousness and a concentration of the mind, heart, will, all the being. If a mantra is found helpful for that, one uses it. OM if rightly used (not mechanically) might very well help the opening upwards and outwards (cosmic consciousness) as well as the descent.
As a rule the only mantra used in this sadhana is that of the Mother or of my name and the Mother's. The concentration in the heart and the concentration in the head can both be used — each has its own result. The first opens up the psychic being and brings bhakti, love and union with the Mother, her presence within the heart and the action of her Force in the nature. The other opens the mind to self-realisation, to the consciousness of what is above mind, to the ascent of the consciousness out of the body and the descent of the higher consciousness into the body.
The name of the Divine is usually called in for protection, for adoration, for increase of bhakti, for the opening up of the inner consciousness, for the realisation of the Divine in that aspect. As far as it is necessary to work in the subconscious for that, the Name must be effective there.
Namajapa has a great power in it.
A hand from some Greatness opened her heart's locked doors
And showed the work for which her strength was born.
As when the mantra sinks in Yoga's ear,
Its message enters stirring the blind brain
And keeps in the dim ignorant cells its sound;
The hearer understands a form of words
And, musing on the index thought it holds,
He strives to read it with the labouring mind,
But finds bright hints, not the embodied truth: