ಓಂ ಯೋ ವೇದಾದೌ ಸ್ವರಃ ಪ್ರೋಕ್ರೋ ವೇದಾಂತೇ ಚ ಪ್ರತಿಷ್ಠಿತಃ

ಓಂ ಯೋ ವೇದಾದೌ ಸ್ವರಃ ಪ್ರೊಕ್ತೋ ವೇದಾಂತೇ ಚ ಪ್ರತಿಷ್ಠಿತಃ | ತಸ್ಯ ಪ್ರಕೃತಿ ಲೀನಸ್ಯ ಯಃ ಪರಸ್ಯ ಮಹೇಶ್ವರಃ ||6||

ಓಂ ಶಾಂತಿಃ ಶಾಂತಿಃ ಶಾಂತಿಃ

ವೇದದಾದಿಯ ಪ್ರಣವ ಶಬ್ಧವು ವೇದದಂತ್ಯದಿ ಅದುವೆ ಸ್ವರವು ಪ್ರಕೃತಿ ಲೀನತೆಗೊಂಡ ತಾರಕನಾತನೇ ಪರಮೇಶರೂಪ ||೬|| ಓಂ ಶಾಂತಿಃ ಶಾಂತಿಃ ಶಾಂತಿಃ

Om - the syllable Om! 

yaH - one who is 

vEdAdau - at the beginning of vedas

swavaraH prOktaH - the swarah that is pronounced, chanted 

vEdantE ca pratishtitaH - that which is established deep within the vedAs or as the conclusion of vedas

tasya prakRitilInasya - of that which is concealed in the prakriti

yaH paraH - that which is beyond 

saH - He

mahEshvaraH - the Lord of lords, Supreme Lord

OmkAra, the eternal sound, which is pronounced at the beginning of the vedas, and that which is established as the ultimate truth at the end of vedAnta (upanishat), one who is beyond the grasps of physical realities (this material world), He is the Supreme Lord Himself!

रसोऽहमप्सु कौन्तेय प्रभास्मि शशिसूर्ययो: |
प्रणव: सर्ववेदेषु शब्द: खे पौरुषं नृषु || Gita 7.8 ||

raso ’ham apsu kaunteya prabhāsmi śhaśhi-sūryayoḥ
praṇavaḥ sarva-vedeṣhu śhabdaḥ khe pauruṣhaṁ nṛiṣhu

ओमित्येकाक्षरं ब्रह्म व्याहरन्मामनुस्मरन् |
य: प्रयाति त्यजन्देहं स याति परमां गतिम् || Gita 8.13||

oṁ ityekākṣharaṁ brahma vyāharan mām anusmaran
yaḥ prayāti tyajan dehaṁ sa yāti paramāṁ gatim

One who departs from the body while remembering Me, the Supreme Personality, and chanting the syllable Om, will attain the supreme goal.

Understanding OM/PraNava:

In Shiva purANa, all the Rishis ask sUta maharshi, 

ṛṣaya ūcuḥ |
praṇavasya ca māhātmyaṃ ṣaḍliṃgasya mahāmune |
śivabhaktasya pūjāṃ ca kramaśo brūhi naḥprabho || 1 ||

1. O lord, tell us the greatness of the syllable Om and that of the six liṅgas, O great sage. Also please tell us the worship of the devotees of Śiva in order.

sūta uvāca |
tapo-dhanair-bhavad-bhiśca samyak-praśnastv-ayaṃ kṛtaḥ |
asyottaraṃ mahādevo jānāti sma na cāparaḥ || 2 ||
[Analyze grammar]

2. All of you, sages, have now requested for a good thing. Only Śiva can explain this properly. No one else.

athāpi vakṣye tamahaṃ śivasya kṛpayaiva hi |

śivo'smākaṃ ca yuṣmākaṃ rakṣāṃ gṛhṇātu bhūriśaḥ || 3 ||

[Analyze grammar]

3. Still I shall explain the same with Śiva’s grace. May Śiva increasingly guard us, you and every one else.

pro hi prakṛtijātasya saṃsārasya mahodadheḥ |
navaṃ nāvāṃtaramiti praṇavaṃ vai vidurbudhāḥ || 4 ||
[Analyze grammar]

4. The syllable Om means an excellent boat to cross the ocean of worldly existence. [Pra=of the Prakṛti i.e. the world evolved out of it. Navam—Nāvāṃ Varam—an excellent boat]

praḥ prapaṃco na nāstivo yuṣmākaṃ praṇavaṃ viduḥ |
prakarṣeṇa nayed-yasmān-mokṣaṃ vaḥ praṇavaṃ viduḥ || 5 ||
[Analyze grammar]

5. Or Praṇava may mean: “there is no world for you” or it may mean “That which leads to salvation”.

sva-jāpakānāṃ yogināṃ sva-maṃtra-pūjakasya ca |
sarva-karma-kṣayaṃ kṛtvā divyajñānaṃ tu nūtanam || 6 ||
[Analyze grammar]

6. Of the Yogis who churn internal knowledge, and those who contemplate within after annihilating the effects of karma, fresh knowledge of divine nature is derived (from within; the knowledge of the self)

tameva māyārahitaṃ nūtanaṃ paricakṣate |
prakarṣeṇa mahātmānaṃ navaṃ śuddhasvarūpakam || 7 ||

7. Through that (divine knowledge), that which is devoid of delusion, the great souls eminently/definitely get the view of the "pure state" (OmkAra enables this) 

nūtanaṃ vai karotīti praṇavaṃ taṃ vidurbudhāḥ |
praṇavaṃ dvividhaṃ proktaṃ sūkṣmasthūlavibhedataḥ || 8 ||

8. The learned state that praNava enables the creation of fresh knowledge. This Praṇava is classified into two (1) the subtle (2) the gross. 

sūkṣmam-ekākṣaraṃ vidyāt-sthūlaṃ paṃcākṣaraṃ viduḥ |
sūkṣmam-avyakta-paṃcārṇaṃ suvyaktārṇaṃ tathetarat || 9 ||

9. The subtle one is to known as a single-syllable, whereas the gross one is of five syllables. In the subtle form, the five syllables are not differentiated.

It goes on and on and you can read more on how OMkara manifests as this material universe and how meditating upon praNava provides the perspective of this world. 

The glorification of the syllable Om and the five-syllabled mantra [Chapter 17] (wisdomlib.org)

Shiva Purana Chapter 17 [Sanskrit text] (wisdomlib.org)