Here Oliver Cowdery initially wrote opposed in the printer’s manuscript; then almost immediately he corrected opposed to oppressed (the level of ink flow for the supralinear insertion is unchanged). Undoubtedly, the orthographic similarity of the two words and the semantic possibility of opposed led to this scribal error. It turns out that there are no examples of oppose being used in the passive in the Book of Mormon text—instead, there are only three active uses:
On the other hand, there are three additional passive examples of oppress in the text as well as three active examples; all six of these are in biblical quotations or refer to such:
The corrected reading in Mosiah 23:12 is at least consistent with other usage in the Book of Mormon.
Summary: Accept “oppressed by king Noah”, Oliver Cowdery’s correction in 𝓟 and undoubtedly the reading of the original text.