He’s Being Followed
As they drove away, everything was fine, and Oliver was delighted to have the rock in his possession. He becomes aware that a dark van is following him. Is this a coincidence? Oliver ensured this by taking the same route three times. The dark van followed him the entire time. This was clearly not a good indication.
He’s Scared
When Oliver hit the gas pedal, the van rumbled instead of speeding up. Oliver was frightened. He considered busting the window and throwing the rock out, but eventually decided against it.