WWW is right. Oil price was at worst the trigger, not the cause.
The cause was twofold: cheap credit extended to people who couldn't afford it even when it was still cheap, and a housing bubble.
The banks screwed up the risk spreading when they valued the loans too highly, and in the process they made such complicated arrangements that it was impossible to judge who actually was at risk. Therefore no-one knew whether any debt-based assets owned by a company would be defaulted on. Therefore suddenly banks didn't know whether other banks could cover their debts, so stopped lending to each other. This was bound to happen as soon as there was any extra pressure on home-owners' budgets, like oil price increases, so those struggling most defaulted.
The banks had to stop giving the cheap credit, and spooked the housing market which burst the bubble, worsening the whole problem by making sure that most of those bad loans would not be recovered by the sale of the house on which they were secured, as they were in negative equity.
This graph gets to the heart of the issue.
