Article LXIII: OM

The syllable, OM, is the representation of Brahman embodied through sound, described as Sabda-Brahman, or God as Sound. Chanting of OM or meditation upon OM is most profound. Although Brahman is indescribable, OM is the closest sound representation of Brahman and can lead one to realize Brahman. Ultimately, OM is Brahman.

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Katha Upanishad

The context of this scripture is a dialogue between Naciketas, a spiritual aspirant, and Yama, the god of death:

II.11: The consummate fulfillment of all desires, the stay of the universe, the endless fruit of all the rites, the bourn of freedom from tear, the most adorable and great, the exalted resort, the basis of life, -- even having seen that, thou being intelligent, O Naciketas, has rejected it with firm resolve.
II.12: The wise man relinquishes both joy and sorrow having realized, by means of meditation on the inner Self, that ancient effulgent One, hard to be seen, immanent, seated in the heart and residing within the body.
II.13: The mortal one who has heard this and comprehended well that subtle principle, the soul of Dharma, after discriminating it (properly), attains it; he verily rejoices having obtained the enjoyable (Atman). Methinks the house is open for Naciketas.
II.14: That which thou beholdest as different from Dharma and Adharma, as different from cause and effect, as different from what had been and what shall be, (please) tell (me) that.
II.15: The goal which all Vedas proclaim, which all penances declare, and desiring which they lead the life of Brahmacharya, -- I tell it to thee in brief -- it is Om.
II.16: This syllable is Brahman; this syllable is also the highest. Having known this syllable, whatever one desires, one gets that.