The preposition "de" here means "from" in the sense of a change "from" one status to another, not intending "separation" from the oppressed, but moving from a source "in" the oppressed. Compare Ovid "Fasti" 5, 616: "inque deum de bove versus erat", "he had been changed from an ox into a god", or Juvenal 7, 197: "fies de rhetore consul", "from an orator you will become a consul". "Oppresso" is the past participle of "opprimere" ("to oppress") in the ablative case as governed by "de", meaning "an oppressed person". The adjective "Liber" is in the nominative case, "a free person".