From the balanced equation, 2 moles of $KClO_3$ produce 3 moles of $O_2$. So, 0.5 moles of $KClO_3$ will produce (3/2) * 0.5 = 0.75 moles of $O_2$. At STP, 1 mole of any gas occupies 22.4 litres. Volume of $O_2$ = 0.75 moles * 22.4 L/mole = 16.8 litres.