The driver of the bus was badly shaken but not injured, Ewers said.
Prosecutor Rachael Woods said the company faced a charge of exposing an individual to harm or illness.
She said the company had inadequate and inappropriate safety measures in place given the risks of sandboarding at the dunes.
Woods said this was a real risk and the company would have been aware of it after a boy was hit by a vehicle on the dunes two years earlier.
The company has pleaded not guilty to the charge.
The trial is set down to last four days. Mr Oh’s son, Sang Kyun Oh, has travelled from Korea for the trial.