Six men and one woman aged 26 to 58 were scooped up Thursday by provincial police in raids in Montreal and Laval and will face charges of robbery and conspiracy.
Two other people were still being sought in the slick theft that targeted oil-recycling companies.
Five vehicles with reservoir tanks and pumps were also seized.
While many people look at leftover cooking oil as something to be washed away by strong detergent, restaurants can clean up by selling it for recycling into such things as biofuels.
Police began investigating the grease ring last May.
Provincial police Sgt. Gino Pare said some people might find used oil to be a weird target.
"At first when we talk about what was stolen, it can seems funny," he said. "We must understand that the impact for the companies can be significant."