Hari” is a word referring to Vishnu, not Shiva. Hara refers to Shiva. Hari or Hara basically means something like “Lord”.
“Om” is the mantra of the creative principle of the Universe and is made up of three sanksrit vowels AUM, symbolizing, in part, beginning, middle, and end (the totality of existence).
“Namah” is means salutations or praise
“Shivaya” is means to Shiva
“Om Namah Shivaya”