Taittirīyopaniṣat 01.19

ॐ इति ब्रह्म। ॐ इतीदं सर्वम्। ओमित्येतदनुकृति ह स्म वा अप्यो श्रावयेत्या श्रावयन्ति। ओमिति सामानि गायन्ति। ओगंशोमिति शस्त्राणि शगंसन्ति। ओमित्यध्वर्युः प्रतिगरं प्रतिगृणाति। ओमिति ब्रह्मा प्रसौति। ओमित्यग्नि होत्रमनुजानाति। ओमिति ब्राह्मणः प्रवक्ष्यन्नाह ब्रह्मोपाप्नवानीति। ब्रह्मैवोपाप्नोति ॥१९॥

oṃ iti brahma। oṃ itīdaṃ sarvam। omityetadanukṛti ha sma vā apyo śrāvayetya śrāvayanti। omiti sāmāni gāyanti। ogaṃśomiti śastrāṇi śagaṃsanti। omityadhvaryuḥ pratigaraṃ pratigṛṇāti। omiti brahmā prasauti। omityagni hotramanujānāti। omiti brāhmaṇaḥ pravakṣyannāha brahmopāpnavānīti। brahmaivopāpnoti ॥19॥

[ॐ (oṃ) - primordial sound; sacred syllable; इति (iti) - thus; ब्रह्म (brahma) - Brahman; absolute reality; ॐ (oṃ) - primordial sound; sacred syllable; इति (iti) - thus; इदं (idaṃ) - this; सर्वम् (sarvam) - all; everything; ओमिति (omiti) - oṃ thus; एतद् (etad) - this; अनुकृति (anukṛti) - imitation; repetition; ह (ha) - indeed; स्म (sma) - verily; वा (vā) - or; अप्यः (apyaḥ) - priests; श्रावयेत्य (śrāvayetya) - makes (someone) hear; श्रावयन्ति (śrāvayanti) - they make (someone) hear; ओमिति (omiti) - oṃ thus; सामानि (sāmāni) - Sāman chants; गायन्ति (gāyanti) - they sing; ओगंशोमिति (ogaṃśomiti) - ओम् + शम् + इति i.e. Om, Peace, thus (it is); शस्त्राणि (śastrāṇi) - hymns of praise; शगंसन्ति (śagaṃsanti) - they recite; ओमिति (omiti) - oṃ thus; अध्वर्युः (adhvaryuḥ) - Adhvaryu priest; प्रतिगरम् (pratigaram) - response; प्रतिगृणाति (pratigṛṇāti) - he accepts; ओमिति (omiti) - oṃ thus; ब्रह्मा (brahmā) - Brahman priest; प्रसौति (prasauti) - he initiates; ओमिति (omiti) - oṃ thus; अग्नि (agni) - Agni; fire; होत्रम् (hotram) - oblation; अनुजानाति (anujānāti) - he permits; ओमिति (omiti) - oṃ thus; ब्राह्मणः (brāhmaṇaḥ) - the knower of Brahman; priest; प्रवक्ष्यन् (pravakṣyan) - about to declare; आह (āha) - he says; ब्रह्म (brahma) - Brahman; absolute reality; उपाप्नवान् (upāpnavān) - has attained; इति (iti) - thus; ब्रह्म (brahma) - Brahman; absolute reality; एव (eva) - indeed; only; उपाप्नोति (upāpnoti) - attains; reaches;]

"Om" is Brahman. "Om" is all this. With "Om" as this imitation, indeed, verily, the priests make (someone) hear, thus they make (someone) hear. With "Om" they sing the Sāman chants. With "Om" and "Sham" they recite the hymns of praise. With "Om" the Adhvaryu priest accepts the response. With "Om" the Brahmana priest initiates. With "Om" Agni permits the oblation. With "Om" the knower of Brahman, about to declare, says, "I have attained Brahman." He indeed attains Brahman.

Taittirīya Bhāshya 01.19

Testimonial from Pancharatra is quoted paraphrasing the same, attributing "Om" to Lord Vishnu because of his superior qualities.

"ॐनामा भगवान् विष्णुः अधिकोच्चगुणत्वतः। यद्यद्रूपं भगवतस्तदिदं सर्वमेव च॥

"oṃ-nāmā bhagavān viṣṇuḥ adhikocca-guṇatvataḥ। yadyad rūpaṃ bhagavatas tad idaṃ sarvam eva ca॥

[ॐ (oṃ) - sacred syllable; नामा (nāmā) - by name; भगवान् (bhagavān) - the Lord; विष्णुः (viṣṇuḥ) - Viṣṇu; अधिकोच्चगुणत्वतः (adhikocca-guṇatvataḥ) - by the superiority of qualities; यद्यत् (yadyat) - whatever; रूपं (rūpaṃ) - form; भगवतः (bhagavataḥ) - of the Lord; तत् (tat) - that; इदं (idaṃ) - this; सर्वम् (sarvam) - all; एव (eva) - indeed; च (ca) - and;]

"The Lord Viṣṇu, known as Oṃ, is supreme due to the superiority of qualities. Whatever forms belong to the Lord, all this indeed is that.

ओमेवाधिकपूर्णत्वात् तस्माद्यज्ञेषु चर्त्विजः। ओमित्येव स्वकर्माणि कुर्वन्त्युद्दिश्य केशवम्॥

omevādhikapūrṇatvāt tasmādyajñeṣu cartvijaḥ। omityeva svakarmāṇi kurvantyuddiśya keśavam॥

[ओम् (om) - the syllable Om; एव (eva) - indeed; अधिक (adhika) - superior; पूर्णत्वात् (pūrṇatvāt) - from completeness; तस्मात् (tasmāt) - therefore; यज्ञेषु (yajñeṣu) - in sacrifices; च (ca) - and; ऋत्विजः (ṛtvijaḥ) - the priests; ओम् (om) - the syllable Om; इति (iti) - thus; एव (eva) - indeed; स्व (sva) - their own; कर्माणि (karmāṇi) - actions; कुर्वन्ति (kurvanti) - perform; उद्दिश्य (uddiśya) - addressing; केशवम् (keśavam) - to Keśava (Krishna);]

Because Om is supremely complete, therefore, in sacrifices, the priests perform their own actions, addressing Lord Keśava, with "Om" alone.

अधिकोच्च शृणुष्वेति हरिमध्वर्युराह हि। अधिकोच्च महाधाम सूच्च देेवेति चाऽदरात्॥

adhikocca śṛṇuṣveti harimadhvaryurāha hi। adhikocca mahādhāma sūcca deve'ti cā'darāt॥

[अधिकोच्च (adhikocca) - in a louder voice; शृणुष्व (śṛṇuṣva) - listen; इति (iti) - thus; हरिमध्वर्युः (harimadhvaryuḥ) - the priest (adhvaryu) addressing Hari; आह (āha) - said; हि (hi) - indeed; अधिकोच्च (adhikocca) - in a louder voice; महाधाम (mahādhāma) - O great abode; सूच्च (sūcca) - proclaim; देवे (deve) - O god; इति (iti) - thus; च (ca) - and; आदरात् (ādarāt) - with respect;]

The priest (adhvaryu) respectfully addressed Lord Hari in a louder voice, saying, 'Listen!' Again, in a louder voice, he said, 'O great abode, proclaim, O god!'

प्रति प्रतिगृणात्येनं शंरूपात्युच्च शावन। इत्याह होता शस्त्रेषु तथैवान्येऽपि चर्त्विजः॥

prati pratigṛṇāty enaṃ śaṃrūpāty ucca śāvana। ity āha hotā śastreṣu tathaivānye'pi cartvijaḥ॥

[प्रति (prati) - towards; in return; प्रतिगृणाति (pratigṛṇāti) - receives; accepts; एनम् (enam) - him; this one; शंरूपात् (śaṃrūpāt) - from auspicious form; उच्च (ucca) - high; loud; शावन (śāvana) - praise; recitation; इति (iti) - thus; so; आह (āha) - says; speaks; होता (hotā) - the priest; the hotṛ; शस्त्रेषु (śastreṣu) - in praises; in hymns; तथैव (tathaiva) - in the same way; likewise; अन्ये (anye) - others; अपि (api) - also; even; च (ca) - and; ऋत्विजः (ṛtvijaḥ) - sacrificial priests; officiants;]

The hotṛ priest, as well as the other officiating priests, receive him with high praise from the auspicious form, saying so in the hymns.

एवं स्वध्यायकृच्चाह ब्रह्मप्राप्त्यर्थमञ्जसा। एवं जानन्त एते तु प्राप्नुवन्त्येव तत्परम्॥"

evaṃ svādhyāyakṛcchāha brahmaprāptyarthamañjasā। evaṃ jānanta ete tu prāpnuvantyeva tatparam॥"

[एवं (evaṃ) - thus; स्व (sva) - own; अध्याय (adhyāya) - study; कृत् (kṛt) - having done; च (ca) - and; आह (āha) - says; ब्रह्म (brahma) - Brahman; प्राप्ति (prāpti) - attainment; अर्थम् (artham) - for the sake of; अञ्जसा (añjasā) - directly; एवं (evaṃ) - thus; जानन्त (jānanta) - knowing; एते (ete) - these; तु (tu) - but; प्राप्नुवन्ति (prāpnuvanti) - attain; एव (eva) - indeed; तत् (tat) - that; परम् (param) - supreme;]

Thus, one who has performed self-study and declares it for the direct attainment of Brahman—such knowers indeed attain that Supreme."

इति च॥

iti ca॥

[इति (iti) - thus; in this way; च (ca) - and / as well;]

- stated thus as well (in Tattvasaṁhitā).