Here in the printer’s manuscript, Oliver Cowdery initially wrote “which was spoken by my father Mosiah”. After some time had elapsed, the of was supralinearly inserted (the correction is in somewhat heavier ink flow). The descender for the f in of makes the f look different from Oliver Cowdery’s usual f, but otherwise the word of does appear to be his, especially the o. In fact, the of here looks quite a bit like the of supralinearly inserted in Mosiah 2:13 (see the discussion there). The of here in Mosiah 2:32 was probably inserted when 𝓟 was proofed against 𝓞. The scribe for the of may be Oliver Cowdery, but we cannot be sure.
Typically, the preference in the Book of Mormon text is for the preposition of to come at the beginning of the relative clause when the verb is speak. But here in Mosiah 2:32, the placement of the of seems quite appropriate. Moreover, the text has a large number of examples with the phraseology “to be spoken of by X”, most of which occur in relative clauses:
In fact, the relative clause in Mosiah 4:14 is very similar in language to the relative clause here in Mosiah 2:32. Indeed, both come from king Benjamin’s discourse. Thus the corrected reading in 𝓟 for Mosiah 2:32 appears to be the original reading.
There are some examples of “spoken by” where the preposition of comes at the beginning of the relative clause, as in Alma 5:21: “him of whom it hath been spoken by our fathers”. Thus one could propose that the original text in Mosiah 2:32 actually read “the evil spirit of which was spoken by my father”. But since the corrected reading in 𝓟 works perfectly fine and is supported by usage elsewhere in the text, the critical text will accept that reading. (For a complete discussion regarding the placement of the preposition in relative clauses, see RELATIVE CLAUSES in volume 3.)
Summary: Accept in Mosiah 2:32 the corrected reading in 𝓟, with the placement of the preposition of immediately after the main verb (“the evil spirit which was spoken of by my father”).