Oligarchy is a form of rule, like monarchy, not an economic system.
Mercantilism is an economic system, like capitalism or socialism.
Mercantilism is an economic system which holds that government should take an active role in protecting domestic industry, through tariffs and subsidies.
Oligarchy, like communism, is BOTH an economic system AND a form of rule. It is an economic system in that government takes measures to protect industries through tariffs and subsidies, but ONLY the particular industries of the oligarchs. We are seeing the beginnings of it here in the US, with the government choosing to prop up certain companies based on whether the choice benefits powerful donors.
To a large extent, it is a mistake to try to separate economic and political systems, in that they are largely interrelated, in that a given political system may be incompatible with a given economic system, and vice versa.