Br.up.5.1.1.S.B--- yadyapi brahma-aatmaadis'abdaa brahmaNaH----- - Although
the words `Brahman', `Atman', etc, are names of Brahman, we understand from the upanishads that Om is Brahman's most intimate appellation. Therefore it is the
best means for the realization of Brahman. It is so in two ways-as the symbol of
Brahman and also as Its name. Just as the image of Vishnu or any other god is
looked upon as the god himself and worshipped, so also, Om is to be looked upon
as Brahman. Brahman is pleased with one who uses Om as an aid, because the
upanishad says-"This is the best help and the highest". (KaTha up. 1.2.17). (See
also Pras'na up.5.2 , MuND. Up. 2.2.6).

    Br.up.5.1.1.S.B-katham omkaaraH brahmanaH pratiikatvena--- saadhanatvena pratipattavya iti. Om is declared to be a symbol of Brahman by the statement `Om
is the ether-Brahman (om kham brahma)'. Now Om is being praised as the Veda,
for the entire Vedas are but Om. They all issue out of it and consist of it. Another
reason why Om is the Veda is that through it one knows whatever is to be known. Therefore, Om being so important, it should be used as the means to Self-realization.

    Taitt.up.1.8.S.B-paraaparabrahmadr.shtyaa upaasyamaana----- pratimeva
vishNoH. Though Om is a mere word, it becomes a means for the attainment of
the supreme Brahman or of hiraNyagarbha, depending on the idea with which it is meditated on. Just as an image is a symbol of Vishnu, Om is a symbol for the
conditioned as well as the unconditioned (apara and para) Brahman.

    Ch.up.2.23.2.S.B-prajaapatiH viraaT kas'yapo vaa----- vyaahr.tayaH. Prajaapati meditated with regard to the worlds, in order to get their essence. Then the three
Vedas flashed in his mind as the essence of the three worlds. He meditated further
on the three Vedas and got as their essence the three vyaahr.tis, bhuuH, bhuvaH,
svaH. Ch.up.2.23.3.S.B--- taani aksharaaNi--- --- omkaaraH sampraasravat tat
brahma. He meditated with regard to the three vyaahr.tis and got as their essence Om. That Om is Brahman. (Thus Om is the essence of all the worlds). Ch.up.2.23.3.
tadyathaa s'ankunaa ----- eva idam sarvam. Just as all leaves are permeated by
the ribs of the leaves, so are all words (speech) permeated by Om. Verily, Om is
all this. MuND. up. 2.2.4 says-Om is the bow, the soul is the arrow and Brahman
is the target. The MaaNDuukya upanishad deals elaborately with Om.

    References to Om are found in S'rii S'ankara's Bhaashya on Bhagavad-gita ch.7.8, ch.8.13, ch.9.17 and ch.17.23,24. Patanjali's Yoga suutra I.27 says that Om is the name
of iisvara.