Here's why:

* Metal oxides: Calcium is a metal. Metal oxides generally tend to be basic.

* Reaction with water: When calcium oxide reacts with water, it forms calcium hydroxide (Ca(OH)₂), which is a strong base.

* pH: Solutions of calcium oxide in water have a pH greater than 7, indicating a basic nature.

In summary, calcium oxide is a basic oxide due to its metal nature and its ability to form a strong base upon reacting with water.