Bhashya
शब्दः अस्या उपासनाया नामकरणं करोति ।उपनिषत्सु एष समयः प्रणवगर्भितप्रणवबहुलोपासनानां महोपनिषदिति नाम तत्र तत्र ।महोपनिषत् उपनिपूर्वस्य सदेः क्विबन्तस्य गत्यवसादनविशरणार्थत्वात् महद्ब्रह्म गमयति ज्ञापयतीति महोपनिषत्, महान्तं संसारमवसादयति क्लेशयति नाशयतीति वा महोपनिषदिति ।'ओमित्यात्मानं युञ्जीतैतद्वै महोपनिषद्म्' इति श्रुतेः ।य एताम् उक्तप्रकारेण प्रतिपादितां महोपनिषदं वेद उपास्ते, स उपासकः कृतपुरश्चरणः कृतं प्रागुक्तोपासनं येन स तथोक्तः महाविष्णुर्भवति ।विष्ळु व्याप्तौ ।द्विरभ्यासः प्रथमोपनिषत्समाप्तिं द्योतयति ॥
In plain words
This is the word that gives the meditation its name. Across the Upaniṣads, wherever a meditation is built around the syllable Om -- carrying it inside, saturated with it -- it earns this same title: the Great Secret, Mahopaniṣad. The name comes from the root sad, set behind upa and ni, and that root carries three senses at once -- to go, to sink down, to come loose. Read one way, it 'leads you to the great Brahman, makes it known' -- that is why it is called the Great Secret. Read the other way, it 'brings the vast wheel of saṃsāra down, wears it away, destroys it' -- and that too earns the name. Scripture itself says it: 'Let a person yoke the self to the syllable Om -- this, truly, is the Great Secret.' Whoever knows this Great Secret exactly as it has been taught, and lives with it in meditation -- having already done the groundwork the practice calls for -- becomes Mahāviṣṇu, the Great Pervader: viṣṇu means simply 'to pervade'. And the doubled word you see here is not an accident -- it marks the close of this first section.
Close to the Sanskrit
The word does the naming of this meditation. In the Upaniṣads, this is the convention: the name 'Mahopaniṣad' [is given], here and there, to meditations that are embedded with the Praṇava and abundant in the Praṇava. 'Mahopaniṣad': from the root sad, preceded by upa and ni, formed with the affix kvip, on account of having the senses of going, sinking-down, and loosening -- 'it leads to, it makes known, the great Brahman' -- thus 'Mahopaniṣad'; or, 'it makes the great saṃsāra sink down, afflicts it, destroys it' -- thus 'Mahopaniṣad'. From the Śruti: 'One should yoke the self with the syllable Om -- this indeed is the Mahopaniṣad.' Whoever knows and meditates upon this Mahopaniṣad as expounded in the manner stated -- that meditator, one 'by whom the aforesaid meditation has been performed', so termed 'one who has done the preliminary practice' -- becomes Mahāviṣṇu. [The root] viṣḷu [means] 'to pervade'. The doubling signals the completion of the first section.