The reason why this is the equation is because Calcium is a +2 cation. Oxygen is -2. When they are equal, it's like opposite attract; which equals CaO.
The ionic formula for calcium oxide is simply CaO. The Calcium ion is an Alkaline earth metal and wants to give up the 2 s orbital elections and become a +2 cation. Oxygen has six valence electrons and is looking to gain two electrons to complete the octet (8) electron count in the valence shell making it a -2 anion.
