what is oum ? why put forward oum in sanskrit mantra ?

                                               I bow to ultimate god shree ganesh


 oum is an absolute ultimate lord .It has three phases. The first ' अ ' (Birth - Lord Brahma), the second 'उ '  (life ) Lord Vishnu), third ' म ' (death - Lord Shiva). These three traits are the properties of the ultimate sovereign. and also the  nature of ultimate God. It also has the nature of Lord Ganesha. The merger of the three Lord of the Lord Sri Ganesh's body.It is written first of all mantra . Written in the ancient mythology, the oum word is immortal. Or the accent after a time it always in some form is traveling in space.So it written next to cast mantra, because it gives power to the same mantra.As the mantra "oum namah Shivaya" means of -I bow to Lord Shiva as the supreme Lord, the god of death and destruction.