extortion: the wrong of using power over someone weaker to squeeze gain out of them, by force, by fraud, or by withholding what is owed, and the goods so taken
Oppression here names a specific wrong, not cruelty in general. Someone with the upper hand, a creditor, an employer, a ruler, a stronger neighbor, wrings gain out of someone who cannot easily push back. The wrong is usually about money or property: forcing a bad bargain, defrauding a partner, or holding back the wages of a hired worker (Leviticus 6:4; Ezekiel 22:29).
The word can point either at the deed or at its spoils. Most often it is the practice itself, the thing the prophets condemn when they say a city is "full of oppression" (Jeremiah 6:6) or that its people "have used oppression" (Ezekiel 22:29). But it can also name the loot: in Leviticus 6:4 a guilty person must hand back "the oppression," meaning the very goods he extorted.
It sits close to open robbery (גָּזֵל), the seizure of property by naked force, and inside the broader idea of חָמָס, violence and wrongdoing of every kind. It is the noun that matches the verb "to oppress" (עָשַׁק): where the verb names the action, this word names the wrong as a thing, whether the practice or the plunder.