In general, the nature of metal oxides is:

Metals combine with oxygen to form metal oxides. Metal oxides are basic in nature.

 Most of the metal oxides can be dissolved in water to form metallic hydroxides and furnish OH^- ions in the solution. 

For example, sodium reacts with oxygen to form sodium oxide which again is dissolved in water to form sodium hydroxide.

$$\begin{gathered} 4\mathrm{Na}+{\mathrm{O}}_2\to 2{\mathrm{Na}}_2\mathrm{O} \\ {\mathrm{Na}}_2\mathrm{O}+{\mathrm{H}}_2\mathrm{O}\to 2\mathrm{NaOH} \end{gathered}$$

 Their pH value is more than 7. So, they are basic in nature.