48 g of individual oxygen atoms = 3 moles of oxygen atoms.
However, oxygen gas is a diatomic molecule (O2).
Since there are 2 oxygen atoms in each molecule of oxygen gas, the molar weight is twice that of an individual oxygen atom (thus the molar weight of oxygen gas is 32g instead of 16g).
So, 1.5 moles of oxygen present in 48g of oxygen gas (because 48/32=1/5).