Omkareshwar is a Hindu temple dedicated to God Shiva. It is one of the 12 revered Jyotirlinga shrines of Shiva. It is on an island called Mandhata or Shivapuri in the Narmada river. The shape of the island is said to be like the Hindu symbol(AUM).
As per Hindu legend, Vindya, the deity controlling the Vindyachal mountain range was worshipping Shiva to propitiate himself from the sins committed. He created a sacred geometrical diagram and a Lingam made of sand and clay. Shiva was pleased with the worhip and believed to have appeared in two forms, namely Omkareshwar and Amaleswara. Since the mud mound appeared in the form of Om, the island came to be known as Omkareswar. There is a shrine for Parvati and five-faced Ganapathi in the temple.
The second story relates to Mandhata and his son's penance. King Mandhata of Ikshvaku clan (an ancestor of Lord Ram) worshipped Lord Shiva here till the Lord manifested himself as a Jyotirlinga. Some scholars also narrate the story about Mandhata's sons-Ambarish and Muchukunda, who had practiced severe penance and austerities here and pleased Lord Shiva. Because of this the mountain is named Mandhata.
The third story from Hindu scriptures says that once upon a time there was a great war between Devas and Danavas(demon), in which Danavas won. This was a major setback for Devas and hence Devas prayed to Lord Shiva. Pleased with their prayer, Lord Shiva emerged in the form of Omkareshwar Jyotirlinga and defeated Danavas.
Adi Shankara's Cave - Omkareshwar is said to be the place where Sri Adi Sankara met his Guru Govindapada in a cave. This cave can be found even today just below the Shiva temple where an image of Adi Shankara has been installed.