Is it permissible to work in the quality control department of a cooking oil factory if the product is packaged at a weight less than what is advertised?

Whoever witnesses a wrongdoing must change it according to their ability. What you mentioned about the reduced weight of oil packaging is an act of shortchanging and deception. Allah the Almighty says: ﴿Woe to those who shortchange [others], Who, when they take a measure from people, take in full, But when they measure out to them or weigh for them, they cause loss.﴾ [Al-Mutaffifin: 1-3]. And the Prophet, peace and blessings be upon him, said: "Whoever cheats is not from me."

Your duty is to advise and, if they do not respond, to report the matter to the authorities, and to be patient in the face of harm. If your task is to monitor the weight, then you bear the sin and the rights of those harmed if you do not prevent it.