cheap oil is not the cause of the market crash, but rather they are both caused by the same problem - fear of recession/depression. Cheap oil (along with many other commodities) have been falling quickly over the past 15 months. Increasing supply combined with "China contagion", ie lower demand growth, have caused inventories to rise. In this case, oil becomes a proxy for "demand", and since it keeps building, that means global economies are not growing. This is further exacerbated by quantitative easing policies that encouraged investment in the past 7 years where-ever there might have been any sort of return. This kept the economy afloat, but it could be argued that it was keeping a zombie economy afloat. These policies are now coming home to roost - over-investment led to oversupply now meets lack of demand.