Oil, or more precisely petroleum, is a naturally occurring black substance within the earth's crust that has been refined into different fuels by humans since 1847 when James Young first refined kerosene from petroleum.
Crude oil is a fossil fuel comprised mainly of hydrocarbons that when burned release carbon dioxide (CO2) into the atmosphere. A process that has led to major anthropocentric (human caused) climate change.
Over the coarse of the past few hundred years we have massively increase our oil consumption in order to fuel our economies, our lifestyles, our transportation, and many other industrial and consumer related products and services. When oil was first being industrially recovered and used, we were experiencing roughly 100 to 1 energy return on energy invested. Today, for each calorie of energy expended to find and recover ever more difficult to access oil deposits, such as those offshore, or much deeper underground, we are only recovering six calories of energy. That is only 6 to 1 today versus 100 to 1 only a few lifetimes ago.
(What Is Crude Oil? by Alexis Madrigal – The Atlantic)
Crude oil, its derivatives, recovery and refinement are some of the most significant sources of carbon dioxide production and climate change sources. The vast majority of scientific experts in climate science have concluded that we must significantly curb our reliance on carbon rich energy sources in order course correct our collective bearing towards a model that is capable of being sustainable. In order to accomplish that we should be diverting oil based energy for wind, hydro, solar, biomass, geothermal, nuclear power, and others.