You haven't mentioned specifics of how the parents have been abusive or oppressive.
Unless you're being beaten silly or sexually abused or having some other extreme form of mental or physical pain meted out on you - and provided that your parents are taking care of your physical needs, i.e. feeding you, giving you a roof over your head, keeping you warm, clothing you etc., then doing this: "let their parents know they won't tolerate oppression" is a bad idea. It's unappreciative and rude and smacks of entitlement attitude. I repeat, I don't know your specifics because you haven't stated, but respect for parents is very important in Islam and you shouldn't let
relatively minor things turn you against them to such a degree.
Until you become capable of standing on your own feet, you are under their care. They have responsibility over you and a position of authority which you should respect.
Is it permissible for a child to make dua to Allah (swt) against their parents if they're oppressive?
If you are going to make dua, why make it a bad one? Why not make dua that Allah opens their heart and gives them understanding? When you make dua for your brother in his absence, angels make the same dua for you. It's mutually beneficial to make a good dua for someone.