Omkareshwar Temple is dedicated to God Shiva. Omkareshwar Temple is one of the 12 jyotirlinga of Lord Shiva, the holiest of holy shrines of Lord Shivas. Omkareshwar is undoubtedly amongst the 12 Jyotirlingas but another name often comes up in this context is Mammaleshwar. Not only the name but the existence of these two are also thoroughly different. The temple of Maleshwar is situated south of the river Narmada, but both the forms of Shiva have been counted as one.
Omkareshwar, the sacred island, shaped like the holiest of all Hindu symbols, ‘Om', has drawn hundreds of generations of pilgrims. Here, at the confluence of the rivers Narmada and Kaveri, the devout have gathered to kneel before the Jyotirlinga (one of the twelve throughout India) at the temple of Shri Omkar Mandhata.
The Temple of Lord Omkareshwar is situated on the banks of River Narmada, in between Indore & Khandwa.Lord Omkareshwar Jyotirlinga Temple is situated by the banks of Narmada River in the Malva area. Gods of all the Gods Lord Shiva's Omkareshwar linga is situated on the mountain Mandhata. Omkareshwar Jyotirlinga located on the south bank of Narmada River on Omkar Mountain an island in mid Narmada.
Before entering the temple, one has to pass through 2 rooms. The Omkareshwar is not affixed to the ground, but is naturally installed there. There is always water around it. The significance of this Linga is that the Linga is not situated below the cupola. The idol of Lord Shiva is situated on the top of the temple. A huge fair is organised here on the day of Kartik Poornima.
Mahmud of Ghazni had invaded and severely damaged the temples at Omkareshwar in the 11th century. Peshwa Baji Rao II had to build a new one as the earlier ones were mutilated beyond repair.
Omkareshwar temple is built in the North Indian style of architecture, with high spires The soft stone of which it was constructed has lent its pliable surface to a rare degree of detailed work, of which the frieze figures on the upper portion is the most striking with the stone roof of the temple being intricately carved also. Encircling the shrine are verandahs with columns which are carved in circles, polygons and squares. The temple also houses the shrines of Panchamuga Ganesha, and Annapoorani which are considered very auspicious by the devotees.
One, who worship the Omkareshwar, will obtain all the fruits desired. There is no doubt in this.
Legend as per Shiva Purana:
It is said that once upon a time Vidhya Parvat practiced severe penances and worshipped Parthivarchana along with Lord Omkareshwar for nearly six months. As a result, Lord Shiva was pleased and blessed him with the desire boon. On the sincere request of all the gods and the sages Lord Shiva made two parts of the ling. In one he resided as the Pranav or Omkar and assumed the form of Omkareshwar and Bhagwan Sadashiva, Parameshwar, Amareshwar or Mamaleshwar assumed form from the Partivlinga.
Once, the holy sage Narada to dispel the pride of Vindhya informed him that everything was present in you. But Meru is loftier. He is classified among gods. But you, never. Vindhya became distressed and he went to the place where Oṃkara himself was present. There he made an earthen idol of Shiva and continuously for six months he propitiated Shiva. Shiva was delighted and bestowed the boon upon him that he would be greater than the Meru Mountain, which is much revered among Hindus. The phallus image of Shiva known as Oṃkara is twofold, Omkareshwar and Mammaleshwar.
The 2 sons of the sun dynasty Mandhata (Ambarish and Mucchkund), practiced severe penance and austerities here pleased Lord Shiva. They had also performed great religious sacrifices here, because of this the mountain is named Mandhata.
Reference: Shiva Purana, Koṭirudra Saṃhita, Chapters - 18