When it is said that the Dua of the oppressed will be accepted, does that mean if an enemy harms you, any Dua you make on that person will be accepted, even if it is negative?
if an enemy harms you
I support Ferdi's answer, that 'yes' the Du'a of the oppressed will be answered.
But it is highly advisable to follow the following verses,
And the retribution for an evil act is an evil one like it, but whoever pardons and makes reconciliation - his reward is [due] from Allah . Indeed, He does not like wrongdoers. (Qur'an Ch. 42, V 40)
And if you punish [an enemy, O believers], punish with an equivalent of that with which you were harmed. But if you are patient - it is better for those who are patient. (Qur'an Ch. 16, V 126)
Tell those who believe to forgive those who hope not for the days of Allah; in order that He may requite folk what they used to earn. (Qur'an Ch. 45, V 14)
So be patient with gracious patience. (Qur'an Ch. 70, V 5)
May the creator guide us all.
The Prophet (ﷺ) sent Mu'adh to Yemen and said: "Be afraid from the curse of the oppressed as there is no screen between his invocation and Allah."
The Prophet (ﷺ) said: "Fear the supplication of the oppressed, it ascends to the sky as if it were a spark."
"The supplication of the oppressed is answered, and if he is an immoral person, then his immorality is against himself."
"[...] and fear the supplication of the oppressed, for it will be answered"
"Beware of the supplication of the oppressed, even if he is an unbeliever, for there is nothing to veil it, besides Allah Almighty."
With all of those narrations it shows that if you do somebody injustice, and the victim does the supplication, it will be answered in one way or another.
Yes, it will be accepted even it's negative.
Here is the hadith : Narrated Abu Ma
(the slave of IbnAbbas) Allah's Messenger (ﷺ) said to Mu`adh when he sent him to Yemen, "You will go to the people of the Scripture. So, when you reach there, invite them to testify that none has the right to be worshipped but Allah, and that Muhammad is His Apostle.
And if they obey you in that, tell them that Allah has enjoined on them five prayers in each day and night. And if they obey you in that tell them that Allah has made it obligatory on them to pay the Zakat which will be taken from the rich among them and given to the poor among them. If they obey you in that, then avoid taking the best of their possessions, and be afraid of the curse of an oppressed person because there is no screen between his invocation and Allah."