Bhashya
अत्रैके वर्णयन्ति-पूर्णात् कारणात् पूर्णं कार्यम् उद्रिच्यते ; उद्रिक्तं कार्यं वर्तमानकालेऽपि पूर्णमेव परमार्थवस्तुभूतं द्वैतरूपेण ; पुनः प्रलयकाले पूर्णस्य कार्यस्य पूर्णताम् आदाय आत्मनि धित्वा पूर्णमेव अवशिष्यते कारणरूपम् ; एवम् उत्पत्तिस्थितिप्रलयेषु त्रिष्वपि कालेषु कार्यकारणयोः पूर्णतैव ; सा च एकैव पूर्णता कार्यकारणयोर्भेदेन व्यपदिश्यते ; एवं च द्वैताद्वैतात्मकमेकं ब्रह्म ।यथा किल समुद्रो जलतरङ्गफेनबुद्बुदाद्यात्मक एव, यथा च जलं सत्यं तदुद्भवाश्च तरङ्गफेनबुद्बुदादयः समुद्रात्मभूता एव आविर्भावतिरोभावधर्माणः परमार्थसत्या एव- एवं सर्वमिदं द्वैतं परमार्थसत्य-सर्वाणि पर्णानि' इत्यादिश्रुत्यन्तरात्; इतश्चायं वेदः ॐका-रः, यद्वेदितव्यम्, तत्सर्वं वेदितव्यम् ओंकारेणैव वेद एने-न; अतः अयमोंकारो वेदः; इतरस्यापि वेदस्य वेदत्वम् अतएव; तस्मात् विशिष्टोऽयमोंकारः साधनत्वेन प्रतिपत्तव्यइति ।अथवा वेदः सः, कोऽसौ? यं ब्राह्मणा विदु. ओ-कारम्; ब्राह्मणानां हि असौ प्रणवोद्गीथादिविकल्पैर्विज्ञेयः;तस्मिन्हि प्रयुज्यमाने साधनत्वेन सर्वो वेदः प्रयुक्तोभवतीति ॥
In plain words
Here is another way some teachers put it: fullness rises out of a full cause. That effect, even now while it still stands as this visible world of two, is genuinely full -- fully real, only wearing the shape of duality. And when the world dissolves, that same fullness is simply taken back into its source, and fullness -- now in the shape of cause -- is all that is left. So through arising, standing, and dissolving alike, there is only ever fullness, whether you call it effect or cause; one single fullness wears two names. That is how Brahman can be one and still show both a dual and a non-dual face. Think of the ocean: it is nothing but water, wave, foam, and bubble together, and the water is real, and the waves and foam and bubbles that rise from it are themselves the ocean's own substance -- they only appear and vanish, but what they are made of is real through and through. In just that way, all this duality... [here the source breaks off and picks back up mid-quotation] '...all the leaves' -- so say this and other scriptures. And this tells us something: the Veda is the syllable Om. Whatever there is to know, you know it entirely through Om alone. That is why Om is the Veda, and why the rest of the Veda even earns the name 'Veda' in the first place. So this Om stands apart, and is to be grasped as the means it is. Or take it another way: that 'Veda' the brahmins know -- what is it? It is Om. To the brahmins it is to be known through its various guises, as praṇava, as udgītha, and so on. Because once Om itself is put to use, the whole Veda is, in effect, already at work.
Close to the Sanskrit
Here, some explain: from the full cause, the full effect rises up. The effect, thus arisen, even in its present state, is indeed full, a really-existent thing, in the form of duality. Again, at the time of dissolution, taking the fullness of the full effect and placing it in itself, fullness alone remains, in the form of the cause. Thus, in all three states -- origination, continuance, and dissolution -- there is fullness alone, of both effect and cause. And that one single fullness is designated differently as effect and cause, by mere difference. And thus Brahman is one, having the nature of both duality and non-duality. Just as, indeed, the ocean itself has the nature of water, wave, foam, bubble, and the rest; and just as the water is real, and the waves, foam, and bubbles arising from it, being themselves of the very substance of the ocean, having only the property of appearing and disappearing, are themselves really existent -- so all this duality [is] really exist[ent] -- [the source text breaks off here and resumes mid-quotation] -- '...all the leaves' -- because of this and other different scriptural passages. And hence this Veda is the syllable Om: whatever is to be known, all of that one knows by this very Om, by this. Therefore this Om is the Veda; the Veda-ness of the rest of the Veda, too, is from this very source. Therefore this Om, being distinguished, is to be understood as a means -- thus. Or else: 'the Veda' -- who is that? [It is] Om, which the brahmins know. For, to the brahmins, that [Om] is to be understood through its various forms, as praṇava, udgītha, and so on. For when that is employed as a means, the whole Veda, in effect, becomes employed -- thus.