Ōṃ is everything says Taittiriya
Sri Garikapati Narasimha Rao in his piece on ‘Mantra Pushpam’
abhrāṇy apaḥ prapadyantē . vidyuth sūryē samāhitā . anavarṇē imē bhūmī .
iyaṃ cāsau ca rōdasī . kim̐svid atrāntarā bhūtam . yēnēmē vidhṛtē ubhē .
viṣṇunā vidhṛtē bhūmī . iti vathsasya vēdanā . irāvatī dhēnumatī
hi bhūtam . sūyavasinī manuṣē daṡasyē .. 1.8.27 ..
The waters ascend to the clouds. The lightning and the sun are established. These earths are without colors (formless). This (earth) and that (heaven) are the two worlds. What exists in between them? That by which both these worlds are held (sustained) — it is Lord Vishnu who sustains the earth. Thus is the knowledge of Vatsa. The earth is abundant with milk and resources, fertile and blessed with grains. It is meant for the well-being of humanity.
vyaṣṭabhnād rōdasī viṣṇav ētē . dādhartha pṛthivīm abhitō mayūkhaiḥ .
kin tad viṣṇōr balam āhuḥ . kā dīptiḥ kiṃ parāyaṇam . ēkō yad dhārayad dēvaḥ .
rējatī rōdasī ubhē . vātād viṣṇōr balamāhuḥ . akṣarād dīptir ucyatē .
tripadād dhārayad dēvaḥ . yad viṣṇōr ēkam uttamam .. 1.8.28 ..
Vishnu supports the two worlds, heaven and earth. He holds the earth firmly with his rays from all sides. What is the strength of Vishnu, they ask? What is His brilliance, and what is his supreme refuge? The one who sustains both the heavens and the earth makes them shine. The strength of Vishnu is said to come from the wind. His brilliance is declared to be imperishable. He supports the worlds with his three strides. That (=air) supreme one of Vishnu is considered the highest.
In the Taittirīya Āraṇyaka 10th prapāṭhaka, 68th anuvāka, we find a beautiful summary of Vedanta:
ōṃ tad brahma . ōṃ tad vāyuḥ . ōṃ tad ātmā . ōṃ tath satyam . ōṃ tath sarvam .
ōṃ tat purōr namaḥ . antaṡ carati bhūtēṣu guhāyāṃ viṡva-mūrtiṣu .
tvaṃ yajn̄as tvaṃ vaṣaṭkāras tvam indras tvam̐ rudras tvaṃ viṣṇus
tvaṃ brahma tvaṃ prajāpatiḥ . tvaṃ tad āpa
āpō jyōtī rasō’mṛtaṃ brahma bhūr-bhuvas-suvar ōm .. 10.68.89 ..
Om is that Brahman, Om is that Vayu, Om is that Self, Om is that Truth, Om is that Everything. Salutations to that Om. It traverses in all entities, hidden, in all forms of the world. You are Yajna, you are Vashat, You are Indra, You are Rudra, You are Vishnu, You are Brahman, You are Prajapati. You are that waters.
All devatas are His body organs:
bhūr bhuvas suvar iti vā ētās tisrō vyāhṛtayaḥ . tāsāmu ha smai tāṃ
caturthīm . māhācamasyaḥ pravēdayatē . maha iti . tad brahma . sa ātmā .
aṅgāny anyā dēvatāḥ . bhū riti vā ayaṃ lōkaḥ . bhuva ity antarikṣam .
suvar ity asau lōkaḥ .. 7.5.10 ..
The fourth word (after bhūr bhuvas suvaḥ) is ‘Maha’. It is Brahman. He is the Self. The other gods are His organs.
Chinnajeeyar Swami ‘tan nama ity upasita’:
prajātir amṛtam ānanda ity upasthē . sarvam ity ākāṡē .
tat pratiṣṭhēty upāsīta . pratiṣṭhāvān bhavati . tan maha ity upāsīta .
mahān bhavati . tan mana ity upāsīta . mānavān bhavati . tan nama ity upāsīta .
namyantē’smai kāmāḥ . tad brahmēty upāsīta . brahmavān bhavati .
tad brahmaṇaḥ parimara ity upāsīta .. paryṇyaṃ mriyante dviṣantassapatnāḥ .. 9.10.10 ..
If you worship ‘That’ as ‘nama’ then all the desires will bow down to you. If you reverse ‘mana’ (mind) it becomes ‘nama’ (bow). Basically, if you bow down to That, then all your desires will be fulfilled.