Temples

Rishi Mrikandu and Marudwati did not have a child even after several years of marriage. The couple then decided to do Shiv Upasana and Tapasya to appease Mahadev.
Lord Shiva was happy with their prayers and asked the couple to make a wish. The couple asked Mahadev to bless them with 'Santan Prapti'.

Lord Shiva gave them the boon but had put forward two conditions :

Either their child will be a very obedient one and towards bhakti but will only have a life of 16 years
Or
The Child will be notorious and lazy but will live for many centuries.

The couple were wise and they decided to choose the former option. After some time they were blessed with a male child named 'Markanday'.
Markanday was an ardent Shiva Bhakta and when his 16th year was about to start he went and held the Maha Mrityunjaya shivling requesting Mahadev to save his life from Kaal.

Lord Shiva obliged his prayers and the Kaal who had come to take back Markanday to Yam Loka was defeated by lord Shiva and he came to be known as ' Maha Mrityunjay' i.e the one who obtained victory.

There was a Rishi named 'Mandhata' who was doing Dhyana to appease Lord Shiva at Kailash. Lord Shiva gave him darshans and blessed Rishi Mandhata but also directed him to go to Kashi and perform his Dhyan there.

On Makar Sankranti in Kashi, Rishi Mandhata had prepared 'Khichadi' Bhog to do Bhojan sewa to Sadhu and Sanyasis. Rishi Mandhata worked hard for the entire day, and managed to cook the food.

After feeding all the Sanyasis, Rishi Mandhata himself sat down to eat the remaining portion of Khichadi Bhog. Just at the same time, Rishi Mandhata saw a weak and old monk coming to eat Bhojan, due to which he was in a dilemma about whether he should give the remaining food to the old Sanyasi or take it himself.

Rishi Mandhata decided to divide the remaining portion of Khichadi into two equal halves, between him and the old Sanyasi. But in reality it was not an old Sanyasi, but it was Lord Shiva himself who had come to give him darshans.

Lord Shiva gave darshans Rishi Mandhata from the Khichadi Bhog itself along with the entire Shiva family i.e. Gauri, Ganesh and Kartikeya. After darshan, Rishi Mandhata asked God for this wish that he would give him darshan in the same form always. Lord Shiva blessed and obliged Mandhata Rishi with his wish. And the divided 'Khichadi Bhog' turned into a self-manifested Shiva Linga half of which is inhabited by 'Maa Gauri' and the other half by 'Kedarnath'.

This self manifested divine Shiva Ling is ardhanarishwar in nature and is also named 'Gauri Kedareshwar'.

When Mata Parvati inquired about the Omkareshwar Shivling, then Lord Shiva narrated a story from Kashi Khanda, Chapter 73.
Kashi is a holy place that is graced by many such Shiva Lingas. Lord Brahma once performed a terrible penance for many years, and an extremely powerful energy came out from below, entered the ground, and spread pure light all around. Lord Brahma saw the divine energy representing the three Vedas (Rig, Yajur, and Sama Vedas) in the form of a divine shivling. Lord Brahma saw the energy of Om, which today rules the entire universe, in that heavenly light, which stood for Akara, Ukara, and Makara. Lord Brahma saw a demon with four horns, seven arms, and two heads. He also saw a divine power that contained all the Vedas, Puranas, and the five Brahmans (Akara, Ukara, Makara, Nada, and Bindu).

The Panchakshara Linga (Panch+Akshara, which stands for the five syllables given in the titles above) mesmerized Lord Brahma so much that he started praising Lord Shiva whom he could see standing in front of him. The praises that Lord Brahma devotedly offered to Lord Shiva are described in great detail by the Kashi Khanda. Lord Shiva granted a divine blessing to Lord Brahma in response to his earnest prayers. Lord Brahma wanted Omkareshwar Mahadev to be the name of the Shiva Linga, which manifested as divine energy, and that Lord Shiva should always reside there. Accordingly, Lord Shiva granted permission and decided that the Shiva Linga would be known as the Shiva Panchayats and would serve as a representation of Aakar, Ukar, Makara, Nada, and Bindu.