A selloff that dragged crude prices near their lowest levels in four months has left many oil drillers, energy traders and fuel producers with the same question: What will OPEC do?

Benchmark U.S. crude has posted three weekly losses in a row, tumbling as much as 20% below its September high, before stabilizing Friday at $77.17 a barrel. Still, the market is flashing signals that suggest some traders are betting the declines will continue.