Oppressors are a real thing and they can oppress a sheep for a lifetime.
(Job 15:20 [KJV]) The wicked man travaileth with pain all his days, and the number of years is hidden to the oppressor.
Oppressors are generally not believers.
Then Asa was wroth with the seer, and put him in a prison house; for he was in a rage with him because of this thing. And Asa oppressed some of the people the same time. (II Chronicles 16:10 [KJV])
(II Chronicles 15:17 [KJV]) But the high places were not taken away out of Israel: nevertheless the heart of Asa was perfect all his days.
Though we have the case with Asa, who oppressed some of the people (2 Chron 16:10) and still had his heart perfect all his days (2 Chron 15:17). But we I guess we could explain that with that he oppressed the wicked since it says in 2 Chron 16:10 that he was wroth with the seers. So he was oppressing those who were an abomination to the Lord I guess.
(Psalms 54:3 [KJV]) For strangers are risen up against me, and oppressors seek after my soul: they have not set God before them. Selah.
We should do judgment and righteousness.
(Psalms 119:121 [KJV]) I have done judgment and justice: leave me not to mine oppressors.
We should deliver the spoiled out of the hand of the oppressor.
(Jeremiah 22:3 [KJV]) Thus saith the LORD; Execute ye judgment and righteousness, and deliver the spoiled out of the hand of the oppressor: and do no wrong, do no violence to the stranger, the fatherless, nor the widow, neither shed innocent blood in this place.
We should not envy oppressors and choose none of his ways. This could have to do with that an oppressor is not a believer. It also seems to insinuate that oppressing in an abomination to the lord (Prov 3:32).
(Proverbs 3:31-32 [KJV])  Envy thou not the oppressor, and choose none of his ways.  For the froward is abomination to the LORD: but his secret is with the righteous.
Because of the oppressors better off is he that has never been than he that is dead and better of is he that is dead than he that is alive.
(Ecclesiastes 4:1-3 [KJV])  So I returned, and considered all the oppressions that are done under the sun: and behold the tears of such as were oppressed, and they had no comforter; and on the side of their oppressors there was power; but they had no comforter.  Wherefore I praised the dead which are already dead more than the living which are yet alive.  Yea, better is he than both they, which hath not yet been, who hath not seen the evil work that is done under the sun.
God is against the oppressors and he will break them in pieces eventually.
(Psalms 72:4 [KJV]) He shall judge the poor of the people, he shall save the children of the needy, and shall break in pieces the oppressor.
Men can be oppressed by children.
(Isaiah 3:12 [KJV]) As for my people, children are their oppressors, and women rule over them. O my people, they which lead thee cause thee to err, and destroy the way of thy paths.
God can delivered to your tormentors (Mt 18:34) if you do not forgive a brother. Tormentor sound similar to oppressor. Notice that forgiving a brother doesnt mean going back in fellowship with him necessarily. It could just mean not holding a grudge towards him. You can forgive and disfellowship.
(Matthew 18:21-35 [KJV])  Then came Peter to him, and said, Lord, how oft shall my brother sin against me, and I forgive him? till seven times?  Jesus saith unto him, I say not unto thee, Until seven times: but, Until seventy times seven.  Therefore is the kingdom of heaven likened unto a certain king, which would take account of his servants.  And when he had begun to reckon, one was brought unto him, which owed him ten thousand talents.  But forasmuch as he had not to pay, his lord commanded him to be sold, and his wife, and children, and all that he had, and payment to be made.  The servant therefore fell down, and worshipped him, saying, Lord, have patience with me, and I will pay thee all.  Then the lord of that servant was moved with compassion, and loosed him, and forgave him the debt.  But the same servant went out, and found one of his fellowservants, which owed him an hundred pence: and he laid hands on him, and took him by the throat, saying, Pay me that thou owest.  And his fellowservant fell down at his feet, and besought him, saying, Have patience with me, and I will pay thee all.  And he would not: but went and cast him into prison, till he should pay the debt.  So when his fellowservants saw what was done, they were very sorry, and came and told unto their lord all that was done.  Then his lord, after that he had called him, said unto him, O thou wicked servant, I forgave thee all that debt, because thou desiredst me:  Shouldest not thou also have had compassion on thy fellowservant, even as I had pity on thee?  And his lord was wroth, and delivered him to the tormentors, till he should pay all that was due unto him.  So likewise shall my heavenly Father do also unto you, if ye from your hearts forgive not every one his brother their trespasses.
How should we deal with an oppressor
We should not envy an oppressor and choose none of his ways.
We should do judgement and rightiousness. We should deliver the spoiled out of the hand of the oppressor.
We can treat the oppressor like the evil man.
If you are under an oppressor it could be because of that you do not love the law (Psalms 119:121, Matthew 18:21-35) so then love the law.