Oxygen is an element and elements have atoms. Oxygen gas is a compound consisting of molecules of oxygen, with each molecule consisting of two oxygen atoms (chemically represented as `O_2` ). The molecular weight of oxygen gas is 32 (= 2 x 16) gm/mole.

Now, each mole of a substance contains an Avogadro's number (or, `6.023 xx 10^(23)` ) of particles. In other words, 1 mole of oxygen would contain `6.023 xx 10^(23)`  molecules. Here, we are given with 16 g of oxygen. The number of molecules in 16 gm of oxygen are 0.5 (= 16/32 moles). 

Thus the number of molecules in 16 gm or 0.5 mole of oxygen gas are 0.5 x 6.023 x 10^(23) = 3.012 x 10^(23) molecules.

Similarly, 16 gm of elemental oxygen (represented as O) would contain 1 mole of atoms, since the atomic mass of oxygen is 16 and hence will have 6.023 x 10^23 atoms.

Hope this helps.