Firefighters, police and rescuers were battling the huge flames at the Saipol factory on Saturday.
The cause of the blast in one of the oil tanks was not immediately clear but a police investigation has been launched.
Two bodies were found at the scene and about 11 people were injured, mostly suffering shock, Dieppe local official Jehan-Eric Vinclair said.
Avril, the company that owns the factory, is France's leading manufacturer of vegetable oils which are used in food products and for biofuels and other industrial products.
It insists that the usual safety procedures had been observed.
Forty people are employed at the site, which is in the port facing the town centre.
The maintenance workers were employed by a subcontractor.
"The risk of pollution is under control, it seems," the town's Communist former mayor, Sébastien Jumel, said on Saturday afternoon.