The works of Ādi Śaṅkarācārya

Bṛhadāraṇyakopaniṣadbhāṣyam

Verse 367

ओमित्येतदक्षरमिदं सर्वमिति ।

In plain words

Om: this one syllable is everything there is.

Close to the Sanskrit

'Om — this syllable is all this' — so it is said.