Calcium oxide reacts with water to form calcium hydroxide:

CaO + H2O -> Ca(OH)2

Calcium hydroxide then reacts with hydrochloric acid to form calcium chloride:

Ca(OH)2 + 2HCl -> CaCl2 + 2H2O

Therefore, the overall reaction is:

CaO + H2O + 2HCl -> CaCl2 + 2H2O

Which simplifies to:

CaO + 2HCl -> CaCl2