KFC Roslyn team member Dave Cannon said he went to the restaurant for a meal before his night shift started.
The fire was burning in the engine at the rear of the Go Bus Transport vehicle.
Mr Cannon grabbed a large fire extinguisher and doused the blaze in the bus, which had stopped.
An ambulance attended and a man and a woman were assessed by St John staff for smoke inhalation, he said.
A passenger on the bus, Vickie Hogg, said she inhaled smoke and was assessed by St John staff.
"I was sitting in the middle of the bus but I was the last one out because I didn’t know what was happening and then it was whoosh, there was smoke through the bus."
St John spokesman Ian Henderson said two patients were assessed and both declined an offer to be transported to Dunedin Hospital for treatment.
Roslyn Fire Station officer Michael Harrison said when crew arrived the fire had been extinguished by KFC staff but there was "smouldering debris" in the engine bay.
The large amount of oil spilt required a section of one lane of Stuart St to be closed.
Firefighters helped to manage traffic flow.
Dunedin City Council acting transport group manager Richard Saunders said Downer contractors put an absorbent grit on the oil spill before the lane was reopened.
Contractors had planned to remove the grit last night, he said.
Otago Regional Council support services manager Gerard Collings said it was concerning when "something bursts into flames" but he declined to comment on the cause of the fire.
The bus driver would report to the council on the incident in the coming days.
