Taittirīyopaniṣat 01.19

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॥

"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.

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

"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.

omevādhikapūrṇatvāt tasmādyajñeṣu cartvijaḥ। omityeva svakarmāṇi kurvantyuddiśya keśavam॥

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

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

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!'

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

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

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

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

iti ca॥

- stated thus as well (in Tattvasaṁhitā).