Here in the printer’s manuscript, Oliver Cowdery initially omitted the relative pronoun (thus he wrote “according to the knowledge I have received from Amoron”), but then virtually immediately Oliver supralinearly inserted which (with no change in level of ink flow for the correction). The nonsubject relative pronoun which is optional here in English, which means that Oliver’s correction probably reflects the reading of the original manuscript since there would have been no motivation for him to insert the which except that 𝓞 read that way. Elsewhere in the manuscripts, Oliver frequently omitted the relative pronoun which; for a list of examples, see under Alma 5:3.
Elsewhere in the text, there are a few more examples of knowledge postmodified in this way by a relative clause. In each case, the nonsubject which is there:
The critical text will maintain the which here in Moroni 9:7.
Summary: Maintain Oliver Cowdery’s inserted which in Moroni 9:7, the corrected reading in 𝓟 and the probable reading in 𝓞; both readings, either with or without the nonsubject which, are theoretically possible.