The molar mass of oxygen (O2) is 32.00 g/mol. This means that 3.120 grams of oxygen is equivalent to 3.120 g / 32.00 g/mol = 0.0975 moles of oxygen.

Each mole of oxygen contains 2 atoms of oxygen. Therefore, 0.0975 moles of oxygen contains 0.0975 moles * 2 atoms/mole = 0.195 atoms of oxygen.

Therefore, there are 0.195 atoms of oxygen in 3.120 grams of oxygen gas.