The whole answer is in its Origin. When there was nothing no form time and space everything was in Avyakta Param Shivam whom we worship in Shiva Linga, Due to spanda of Shakti a part of this bliss ocean without a ripple get conscious of his own existence and thought emerged That was first Thought of God OM. Then he manifested himself in Sakara form as Adi Shakti Shiva. They made there abode beyond time. Called Ananda Van, That is True Kashi also, On earth we have its ansha, that is Our Kaashi is also beyond time it survives pralaya Rudra Deva keeps it on his trident. Its only thing which survives Pralaya.
After gazillions of years, Adi Shakti suggested Shiva to make universe to which he said as he is aptakaam why we need that and have cudgels to take care of them, So she said we will make Devas and Gods to do the business. Adi Shiva rubbed amrit on Left hand and came Vishnu, and he asked him to do tapasaya for realization of Param Shivam in Paravaani. Lord Vishnu attained enlightenment and streams of water issued forth hence he called Narayana the one who dwells in water. Then he went in to Yog Nidra by bhagwatis bidding and Shiva rubbed amrit on His right hand and came Bramha, he placed the beej the seed of Bramha in stomach of Vishnu and a lotus sprouted with Bramha sitting on it.
Lord Bramha and Vishnu were at ekarnava singularity that time, Lord Bramha having five heads Looked at Lord Vishnu and woke him up, and said i am Lord God who are you , you shall bow down to me, Lord Vishnu said no i am Lord God you must bow down to me, and they both fought for eons but none could win each other then suddenly eve existing Shiva took form of fire pillar Luminous then trillions of suns, With a great sound heave of sound of OM, between them, and Aksashwani told them the who would find either end or beginning of this Fire Pillar would be greater God.
Vishnu took form of Varaha a boar and went downwards , Bramha took form of swan and went upwards none could find its end till 1000 of years yet bramha while flyig upwards found ketaki flower dropping, he aksed him who you are from where you are falling he said, i dont know since eons i m just falling down, Bramha convinced him to lie that i will say to vishnu that i have found you over top of this fire like pillar,
They met at same place again, and Bramha lied and ketaki flower have false witness Lord Vishnu accepted it and started worshiping Bramha then again with pralayankar sound OM first emerged from Shivalingam, then came Panchakshari Mahavidya, then came gayatri of 24 vyahrtis and seed of 50 crore mantras, and Lord Shiva Manifested in Sakaar form and cursed Bramha not be worshiped and gave boon to Lord Vishnu that he will be worshiped as God by his followers as he was desirous of Godliness, and given him many vardaans and banned ketaki from his worship.
Now upon request of Lord Vishnu and Bramha Shiva gave Bramha Yagna Bhaag that in yagnas he will be worshipped. And ketaki can not be put near him but when his mandal is made it can be put some where outside.
And said to both of them that while you guys were fighting thinking that your are Gods, i giggled a bit seeing my kids fighting and head swayed a bit this ketaki was there and it fell down, then i become this fire pillar, Now you must know that i am final God, no body is above or beyond me. Hence you and all shall worship this avyakta primordial ShivaLingam from which whole universe and you sprout and get merged in it at end. And after some time from my anahad heart my replica Shiva Rudra would come, He would be One among you, you all three are my own parts, and Vishnu you will be equally worshiped as my form Rudra Shiva, you trinity shall be considered one. And you and Bramha shall worship my own form Shiva Rudra and you trinity would have mutual respect and reverence for each other.
And then putting a cover on there heads Shiva gave them Panchakshari Mahavidya and since then worship of Shiva Lingam started.
Now the OM What it is, Precisely Its Unification of all that exists whiech permeats within Pramtama Shiva Including Consciousness.
The best source yet very condensed and really there is no point of bing modest it is meant for highest of Sadhaks the Top Notch, Every Shloka is very condensed and describes Prabramha Shiva Tattva and declares the way to reach Shivatva is Om.
In Muktika Upnishad Hanumanjee asked Shree Ram how to get Kaivalya the only true moksha. Shree Rama said that is only possible through Gyan of Praam Shivam and Kaashi, for the caliber you have the great gyani you already are Oh Hanuman onlly Mandukya Upnisahd can bring one to Kaivalya , See such is Importance of Mandukya and it describes Shivatwam Tureeyateet adwait awastha infinite bliss as finality.
Let us see what special has Mandukya Upnishad, it mainly talks about meaning of Om its sadhna and at alas it declares what you attain,
- अमात्रश्चतुर्थोऽव्यवहार्यः प्रपञ्चोपशमः शिवोऽद्वैत
- एवमोङ्कार आत्मैव संविशत्यात्मनाऽऽत्मानं य एवं वेद ॥ १
That which is partless, soundless, incomprehensible, beyond the senses, blissful, non-dual Shiva the one and that wherein all phenomena are resolved is the “forth,” the Shiva Self . The one who knows It dissolves the self in It.
Though the meaning is wast yet precisely it has five syllables inside it, or Panch vyahratis which means hidden greater meaning of a word. which is explained in this picture
There are Long commentaries being written on its meaning as per the Shlokas one can check on net.
Now dont go into supremisation business, None in Sakaar is Supreme, Supreme is what Nirakar beyond name form as shows the shloka of mandukya upnishad here, The Param Shivam Adwaitam. Whom we worship in Shiva Lingam.
The five vyahritis also mean Bramha Vishnu Mahesh Than Iswara and Maheswara. And the five powers of Parama Shivam, Sristhi Sthiti Laya, Tirobhav and Anugrah.