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The mass of Oxygen atom is 16 g, right!!!
Then, mass of O₂ is 16*2=32 g.
No.of moles= Given mass/ Molar mass
  = 12/32
  = 6/16
  = 0.375

Thank you.
Hope this will help you.

Molar mass of O₂ = 32 g
So 32 gram of O₂ is 1 mole.
12 gram of O₂ = 12/32 = 0.375 mole