Let say Number of moles of oxygen is n then using ideal gas equation.
PV = nRT
3atm x 0.224L = n x 0.082 x 273 k
n = 0.03 mole
As we know
∵ 1 mole oxygen contain = 6.022 x 1023 oxygen molecule
∴ 0.03 moles oxygen contain = 0.03 x 6.022 x 1023
= 1.81 x 1022 oxygen molecule