Allah tells us He is Pure, Loving, and absolutely Just in every respect. He says He is the Best of Judges. He also tells us the life we are in here is a test. He has created everything existing and He created whatever happens as well. There is nothing in this existence except what He has created. He also says in the Quran He created evil (although He is not evil). He is using this as one of the many tests for us.
Consider people who do so much evil in the earth and then live to a ripe old age in the splendor and wealth of their ill-gotten gains and die without ever being taken to task for their deeds. Where is the justice or fairness in this? Allah provides a clear answer for us in Quran as to what is in store for these most evil of people:
A brief enjoyment in this world! - and then unto Us will be their return, then We shall make them taste the severest torment because they used to disbelieve [in Allah, belie His Messengers, deny and challenge His proofs, signs, verses, etc.] [Noble Quran 10:70]
That Day mankind will proceed in scattered groups that they may be shown their deeds.
So whoever does good equal to the weight of an atom, shall see it.
And whoever does evil equal to the weight of an atom, shall see it. [Noble Quran 99:6-8]
Do people think that they will be left alone because they say: "We believe," and will not be tested.
And We indeed tested those who were before them. And Allah will certainly make (it) known (the truth of) those who are true, and will certainly make (it) known (the falsehood of) those who are liars, (although Allah knows all that before putting them to test).
Or those who do evil deeds think that they can outstrip Us (i.e. escape Our Punishment)? Evil is that which they judge!
Whoever hopes for the Meeting with Allah, then Allah's Term is surely coming. And He is the All-Hearer, the All-Knower.
And whosoever strives, he strives only for himself. Verily, Allah is free of all wants from the 'Alamin (mankind, jinns, and all that exists).
Those who believe [in the Oneness of Allah (Monotheism) and in Messenger Muhammad peace be upon him , and do not apostate because of the harm they receive from the polytheists], and do righteous good deeds, surely, We shall remit from them their evil deeds and shall reward them according to the best of that which they used to do.
And We have enjoined on man to be good and dutiful to his parents, but if they strive to make you join with Me (in worship) anything (as a partner) of which you have no knowledge, then obey them not. Unto Me is your return, and I shall tell you what you used to do.
And for those who believe (in the Oneness of Allah and other items of Faith) and do righteous good deeds, surely, We shall make them enter in (the entrance of) the righteous (i.e. in Paradise).
Of mankind are some who say: "We believe in Allah," but if they are made to suffer for the sake of Allah, they consider the trial of mankind as Allah's punishment, and if victory comes from your Lord, (the hypocrites) will say: "Verily! We were with you (helping you)." Is not Allah Best Aware of what is in the breast of the 'Alamin (mankind and jinns)?
Verily, Allah knows those who believe, and verily, He knows the hypocrites [i.e. Allah will test the people with good and hard days to discriminate the good from the wicked (although Allah knows all that before putting them to test)].
[Noble Quran 29:2-11]
Reward and punishment are definitely a part of the Next Life on a very permanent basis. Those who suffer in this life and seemly have nothing to show for all their hard works, good deeds and sacrifices and maintaining the faith can look forward to a far greater reward in a place where these things will not be temporary but rather they will be for eternity. Those who corrupt, violate, injure and do evil and destruction seem to escape any justice here, yet in the Next Life they too will see the fruits of their labors, and certainly they will be paid according to what they have done as well.
As regards oppression, this is something Allah forbids for Himself to do to anyone and He hates it when anyone oppresses someone else. He does have absolute power over everything. He allows sickness, disease, death and even oppression so we can all be tested in what we do.