Verse 5.2

एतद्वै सत्यकाम परं चापरं च ब्रह्म यदोङ्कारः ।
तस्माद्विद्वानेतेनैवायतनेनैकतरमन्वेति ॥ २ ॥

etadvai satyakāma paraṃ cāparaṃ ca brahma yadoṅkāraḥ |
tasmādvidvānetenaivāyatanenaikataramanveti || 2 ||

2. The syllable ‘Om’ is verily the higher and lower Brahman. Therefore, the knower, by this means, surely reaches either of them.

Shankara’s Commentary:

Com.—This Brahman—the higher Brahman, true, undecaying, known as Purusha, and the lower Brahman known as Prana, the first-born—is, indeed, the syllable ‘Om’; for, the syllable ‘Om’ is its Pratika, substitute. The higher Brahman, not capable of being indicated by words, etc., and devoid of all distinguishing attributes, is, therefore, being beyond the reach of the senses, incapable of being comprehended by the mere mind. But to those worshippers who contemplate on the syllable ‘Om’ as upon the image of Vishnu, etc., and regard it as a substitute for Brahman, the Para Brahman is understood to reveal itself, from the authority of the sastras; so too the lower Brahman. Therefore, it is by courtesy, that Brahman, the higher and the lower, is said to be the syllable ‘Om.’ Therefore, he who knows thus, attains either the higher or the lower Brahman, by this very means to the attainment of the atman, i.e., by the meditation on ‘Om.’ For the syllable ‘Om’ is the nearest stay of Brahman.