Calcium reacts with oxygen to form an ionic bond.
An ionic bond by definition is the kind of chemical bond formed when there is a complete transfer of valence electrons from one atom to another, generating two oppositely charged ions.
In ionic bonding, the metal loses electrons to become a positively charged cation, whereas the nonmetal accepts those electrons and becomes a negatively charged anion.
Calcium has two electrons in its valence or outermost shell while oxygen has 6 electrons. Calcium gives up these two electrons and becomes positively charged and oxygen accepts them and becomes negatively charged forming an ionic bond resulting in a new compound called calcium oxide.
