O, S, Se, Te, Po are all elements found in group sixteen of the periodic table. Remember that the commonality between all the elements in the same group of the periodic table is that they all possess the same number of outermost electrons. All elements in the same group must have the same number of outermost electrons in their outermost shell.
Now, having six electrons in their outermost shell implies that they readily bond with two other atoms to complete their octet. In accordance with the octet rule. This explains why oxygen bonds with calcium by gaining two electrons in calcium oxide.
