The oppressor and the oppressed both need help. It is a good deed to help the oppressed get their right in this world. Same way, we will be doing a favour on many others (who could have been wrong in the future) , including the oppressor by showing him/ her the right path. This again is a good deed adding to our scale of goodness.
This is reiterated in Quran too.
And what is wrong with you that you fight not in the Cause of Allâh, and for those weak, illtreated and oppressed among men, women, and children, whose cry is: “Our Lord! Rescue us from this town whose people are oppressors; and raise for us from You one who will protect, and raise for us from You one who will help.” [004:075: Al Quran]
We will reserve the houses of the hereafter exclusively for those who do not seek self-glory in this life and do not cause oppression and corruption to spread. The final outcome belongs to those who fear (Allah). [028:083: Al Quran]
Here is it clear that God calls upon us all to fight for the cause of the oppressed and the wronged people in the world. There are several of these people all around us. And they're waiting for us to be rescued. Abducted children, molested kids, kids forced into illegal acts, some neighbour being ill-treated by in-laws, a orphan being beaten by his own relatives; injustice, corruption and a lot more issues which affect us and others around us....
Apart from all, the 'oppressed' or 'wronged' person has another power, will can destroy the oppressor. The bad words he mutters against his oppressor in his pain and anger.