Taittirīya Upaniṣad 1.8

1.8.1

omiti brahma /
omitīdam̐ sarvam /
omityetadanukṛtirha sma vā apyo śrāvayetyāśrāvayanti /
omiti sāmāni gāyanti /
om̐śomiti śastrāṇi śam̐santi /
omityadhvaryuḥ pratigaraṃ pratigṛṇāti /
omiti brahmā prasauti /
omityagnihotramanujānāti /
omiti brāhmaṇaḥ pravakṣyannāha brahmopāpnavānīti /
brahmaivopāpnoti // 1//
ityaṣṭamo 'nuvākaḥ //

Om is brahma. Om is this whole world.

Om—that is compliance. As also, verily, it is well known—upon the words ‘O! Call forth!’ they call forth. With ‘Om’ they sing the Sāman chants. With ‘Om! Śom!’ they recite the Invocations of Praise (śāstra). With ‘Om’ the Adhvaryu priest utters the Response. With ‘Om’ the Brahman priest (brahma) utters the Introductory Eulogy (pra + √stu). With ‘Om’ one assents to the Agni-oblation (agnihotra). ‘Om,’ says a Brahman (brāhmaṇa) about to recite, ‘may I get the sacred word (brahma)!’ He does get the sacred word.