oppression is always wrong no matter what. In other words it is absolutely wrong. — Fides Quaerens Intellectum
It is essential to realise that "oppression" is not always referring to the same thing. If person A and person B agree oppression is wrong but person A considers X oppression and person B does not then person A and B's views about oppression are not the same. Even if person A & B agree X is oppression, they could disagree on many particulars, these disagreements aren't necessarily insignificant, that's a big part of what makes morality complicated.
Why is oppression seen as universally bad? One explanation is that oppression is an inherently critical description. Even if the oppressing party actually used the term oppression they would also believe that the circumstances call or allow for it and it is justified.
The definition of oppression according to
dictionary.cambridge.org
"a situation in which people are governed in an unfair and cruel way and prevented from having opportunities and freedom"
Keywords are "unfair" and "cruel", oppression cannot be the correct term to describe something fair and reasonable which it usually is from the perspective of those who do the oppressing.
Definition of tyranny by
dictionary.cambridge.com
"government by a ruler or small group of people who have unlimited power over the people in their country or state and use it unfairly and cruelly"
No reasonable definition of tyranny will make it sound like it could possibly be a compliment. The definition of the word makes it so that everyone must see tyranny negatively because tyranny is used to disapprove. If you called Mussolini a tyrant, a pro-Mussolini fascist would either disagree that he was tyrannical or he would argue that the tyranny was net beneficial and justified, likely tweaking the definition a bit.
Essentially every culture has valued loyalty, integrity, strength, fairness, respect, honesty and more. — Fides Quaerens Intellectum
But is it culture that is responsible for these values or human psychology? Dogs care about fairness and they recognise who is the pack leader. They show loyalty and respect to the pack leader. They dislike it when you take things from them and like it when they are given food or toys. They can be taught what behaviour is good and what is bad. They know when is dinner or walk time and expect that consistency. Basically, while dogs are not humans, it's not as though culture has created the importance of values like fairness, fairness is a part of our culture because humans value fairness as a result of their psychology.
Is the dog onto something or have we just evolved in a similar way to dogs? The latter and the biases of our brains should be addressed as biases and not some mysterious interaction between factual morality and our psychological, hormonal and emotional development. Just as what I find attractive or tasty shouldn't be viewed as objectively attractive or tasty.
A decent way to wrap your head around how morality works is to think about what a dog might do. If I have two dogs and I give one dog all the affection and food and the other dog gets jealous or growls and gets angry, yeah, that's pretty much what most children would do as well. If someone were to beat and kick their dog and treated them cruelly, it's going to be hostile towards them, growling and baring their teeth at them, right? Humans will develop the same contempt.
Humans have empathy, where even if you just watched someone kicking their dog, you would be able to put yourself in the dog's position and feel for the dog. That's what separates humans from other animals, the injustice doesn't have to happen to you for you to get angry about it. Now that's almost exactly how morality operates and it's a strong feeling. People may not get merely mildly angry about something like that. They are capable of feeling for the dog's situation so strongly that they could desire to take literal, violent revenge on the dog's behalf, they may actually cry because they find it so sad and awful. I could give examples but I assume you've already seen things like this.
Why is murder wrong? If I watch a video of a parent crying for their deceased child, I, without intent, will likely feel sad or angry on their behalf, that's just the natural consequence of empathy. People don't need a fancy articulation of what makes murder wrong, they just need to hear about how the deceased person had goals and dreams, they just need to see the grief of the relatives and they can feel intense hatred towards the murderer, a person they've never even met before. And because everyone in a community can empathise with the deceased and their loved ones, the murderer is condemned and ostracized by the entire community. Now for you, maybe you decide that murder is objectively wrong, it really doesn't change much.
Anything that you can reasonably be upset about if it happened to you, you can get upset about it when it happens to someone else because of empathy. Morality reinforces itself effortlessly in such circumstances and that's mostly what morality is but other areas of morality still have some other psychological basis that gives potency to any view on right or wrong.
The potency of moral views does not rely on absolutism, it doesn't come from thinking you're objectively correct. What makes murder upset people is not their contempt for a non-factual understanding of morality. People don't get upset because "murder is objectively wrong, you did a wrong thing, that's bad". They get upset due to the raw emotional experience caused by empathy, outraged by the consequences of the act, they'll viciously attack the character of the perpetrator and be appalled and saddened by what happened to the victim.
Do you disagree with that? Can you see how I think Mussolini is wrong because I feel angry for the people he killed, rather than because I'm upset he violated a moral absolute? Can you see that even though I don't think Mussolini violated a moral absolute, that it doesn't impact my anger about all the innocent people he murdered?