Prashna-5-3-sa yadyekamātrama

Sri Shankara’s Commentary (Bhashya) translated by S. Sitarama Sastri

Though he may not know the division of all the mâtras (measures) of the syllable ‘Om’, still he surely reaches the excellent goal by virtue of the meditation on the syllable ‘Om’. One who depends entirely on the syllable ‘Om’ does not, by the defect of a partial knowledge of it, attain evil, as one, who has fallen from both karma and knowledge. Even if he constantly meditates on ‘Om’ knowing it only as one mâtra, he enlightened by that meditation of ‘Om,’ with only one mâtra, soon reaches the earth. What? The world of men; for, many are the births possible in this earth. Of these, the riks take the worshipper only to the world of men. The first only mâtra of the letter ‘Om’ meditated on is the Rig Veda. By that, in his birth as man, he becomes pre-eminent among the twice-born and combining tapas, Brahmacharya and faith experiences greatness, i.e., does not become an unbeliever, acting according to pleasure. One who has fallen from yôga never attains a bad goal.

Note.—By the meditation of ‘Om,’ as one mâtra, some mean the meditation on the letter ‘A’ alone of the syllable. Others again, take it to mean the contemplation on the whole syllable ‘Om,’ but pre-eminence being given only to one mâtra.

Links
Various resources on the Prashnopanishad
Upanishad Series – Kena Upanishad Commentary by Swami Chinmayananda