Here in the printer’s manuscript, the subject of the as-clause takes the pronoun form you, which was changed to the archaic ye by the 1830 typesetter. Generally the Book of Mormon text has ye in subject position and you in object position (just like the King James Bible). But there are some examples of you in subject position in the earliest textual sources, as here in Mosiah 23:12. It is possible that the immediately preceding occurrence of you (“and now I say unto you”) caused an original ye in “as ye have been oppressed by king Noah” to be replaced with you early on in the transmission of the text. However, there are quite a few examples of you being used in subject position in the earliest text (see the statistics cited under Mosiah 4:14); thus the critical text will in each case let the earliest textual sources determine whether the subject pronoun form is ye or you. In subject position, the biblical ye clearly dominates in the text, but you is also possible. For further discussion, see ye in volume 3.
Summary: Restore in Mosiah 23:12 the modern use of you in subject position since the earliest extant textual source (the printer’s manuscript) reads this way (“as you have been oppressed by king Noah”).