Lord Shiva is called Pranavamurthi, as he is believed to be originated from the Pranavanaada. Pranavanaada here is the Omkara. It is believed that Lord Shiva had his origin from the Omkara and then he started creating the Universe and the remaining Gods, thereby assigning their duties.
The Srushti, which is called as the Universe, is formed by the accumulation of all the smaller particles. The energy obtained by these particles is nothing but the Omkara, which has been the origin of all the particles in the Universe. Lord Shiva had originated from the Omkara and was the first to initiate the process of creation. He created all the Universe, creatures, Gods and their activities according to the nature of the beings.
Lord Shiva is the origin of Life and has the sole control over presenting the life and taking off the same from any creature in this Universe. He can even present life to the lifeless and make a creature lifeless.
Shiva created the Nature in this Universe with a beauty that anyone can enjoy the Nature. The Nature is so created by Lord Shiva that all the beings exist and vanquish in the Nature by virtue of their own Deeds and Karma. The Nature takes care of all the beings, their survival, existence, hunger and their prosperity.
Nobody knows the secret behind how the entire Nature and the Universe have been created except Lord Shiva, as he is the Lord of Nature and the Universe. That is the reason; he is called as Sarveswara, which means he resides in every Being. Thus, Shiva is the Pranavamurthi, who had originated from the Omkara.