Electronegativity is a property of atoms or molecules to attract electrons toward it. We can determine that using the electronegative differences of the two compounds. These electronegative differences can determine the polarity of the bond wherein the greater the electronegative difference value, the more ionic the bond is.
Sometimes we often classify the electronegative differences as:
< 0.4 = non-polar covalent bond
0.5 - 1.7 = polar covalent
> 1.7 = ionic
For calcium oxide (CaO) and calcium chloride (CaCl2), the electronegative difference can be computed as:
Ca-O = |1.00 - 3.44|* = 2.44
Ca-Cl = |1.00 - 3.16|* = 2.16
We can see that the value for Ca-O bond is greater than Ca-Cl bond which indicates that Ca-O bond is more polar than Ca-Cl bond. In other words, CaCl2 shows more characteristic of a covalent bond though both of them can be classified as ionic.