To determine the oxidation number of oxygen in CaO, we need to consider the charges of the individual atoms and the overall charge of the compound. Calcium (Ca) has a +2 oxidation number, and oxygen (O) has an oxidation number of -2. Since CaO is a neutral compound, the sum of the oxidation numbers of calcium and oxygen must be zero.

Let x be the oxidation number of oxygen in CaO. Then we can set up the following equation:

(+2) + x = 0

Solving for x, we get:

x = -2

Therefore, the oxidation number of oxygen in CaO is -2.