Here the 1840 edition changed the plural oppressions to the singular oppression. This change could be due to Joseph Smith’s editing, or it may simply represent an error in the 1840 edition. The RLDS text restored the original plural oppressions in the third RLDS edition (1908), probably by reference to 𝓟.
Here in Alma 21:21, either the singular or the plural is possible, although normally in English we expect the singular oppression since the noun is usually a mass noun. But the Book of Mormon text frequently uses nouns in the plural when English speakers would prefer the singular. Consider, for instance, the discussion under 2 Nephi 10:6 regarding the occurrence of the plural bloodsheds in the Book of Mormon text as well as the tendency to replace bloodsheds with bloodshed.
Elsewhere in the text we have three occurrences of the expected singular oppression but none of the plural oppressions:
The first and the third of these are quotes from the King James version of Isaiah (namely, from Isaiah 5:7 and Isaiah 54:14). So in the nonbiblical text of the Book of Mormon, there is one occurrence of the singular oppression (in Helaman 4:12) but also one of the plural oppressions (here in Alma 21:21). The critical text will maintain the original plural oppressions in Alma 21:21 despite its unexpectedness in current English.
Summary: Retain the original plural reading oppressions in Alma 21:21 despite its unique occurrence in the text.