An oxidative insertion, also known as oxidative addition, is an insertion in which one of the reactants is a metal that is oxidized (see oxidation) during the reaction.

eg. 1:

In this insertion, one reactant is a metal, namely, magnesium, and magnesium is oxidized during the reaction.  Thus, this insertion is an oxidative insertion.

eg. 2:

In this insertion, one reactant is a metal, namely, zinc, and zinc is oxidized during the reaction.  Thus, this insertion is an oxidative insertion.