Shiva or Siva? Which is the original name?

Can we say Siva instead of Shiva ? Many say Om nama chivaya, or Siva Siva. Why? Is it correct?  Shiva is a God worshipped by all Hindus. Both North and South Indians lay claim to this deity. How old is His worship? Where did He belong? I have tried to answer this question by delving into the meaning of the word and the antiquity. 0:00 Intro 1:55 Method of Determination 2:44 Shivah meaning 4:59 Shivah Definition 5:45 Shivah etymology 7:54 Reference shooks 8:34 Shivah in Vedas 9:44 Summary Shivah 10:33 Siva in Ancient Tamizh 12:12 Names of ancient Siva 13:32 Siva Etymology 14:49 Shiva Rudra Siva