An oxidation number changes from -2 to 0. Does it gain or lose electrons?

Answer 1

Think of it this way. Let's consider Oxygen, O. Oxygen is neutral so it has an oxidation number of 0, O⁰. However, when it gains two electrons, it has an oxidation number of O^-2. When it loses those same electrons, it will again become neutral, O^o.

So, when the oxidation changes from -2 to 0, the chemical species is losing electrons.