Sri Aurobindo
Letters of Sri Aurobindo
Volume II - Part 2
OM is the mantra, the expressive sound-symbol of the Brahman Consciousness 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. The last is usually the main preoccupation with those who use the 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.