Here's how to determine the number of electrons in an O^2- ion:

* Neutral Oxygen: A neutral oxygen atom has 8 electrons (its atomic number is 8).

* Gaining Electrons: The "-2" charge on the O^2- ion indicates that it has gained two electrons.

* Total Electrons: Therefore, an O^2- ion has 8 (from the neutral atom) + 2 (gained electrons) = 10 electrons.