Mg+O2=MgO Balance the above Chemical equation?

Given Equation is 

$\mathrm{Mg} + O2 \to \mathrm{MgO}$

Balancing the Chemical Equation is nothing but Conservation of Matter. As Matter is Neither created nor Destroyed, it must be conserved.

Here we have 1 Mg and 2 O on the left side and 1 Mg and 1 O on the right side

To balance that, add 1 Mg on the left side so that it becomes 2 Mg and 2 O on the left side

i.e,

$2 \mathrm{Mg} + O2 \to 2 \mathrm{MgO}$

Here it has 2 Mg & 2 O on the left side and 2 Mg and 2 O on the right side and the Chemical equation is Balanced.