Oliver Cowdery initially wrote the relative pronoun that here in 𝓟; then he virtually immediately changed it to who by crossing out the that and supralinearly inserting the who (the level of ink flow is unchanged). Elsewhere in the Book of Mormon, there are three occurrences of upon him that (where that is a relative pronoun, not a conjunction), but there are no other occurrences of upon him who. More generally, there are 17 other occurrences of him who in the original Book of Mormon text (I include here only cases of restrictive who), plus 55 occurrences of him that (a good many of these are in quotes from the King James Bible). Thus there appears to have been no motivation for Oliver to edit the text here in 𝓟 from that to who except to make it agree with 𝓞.
Summary: Retain in Mosiah 2:33 the relative pronoun who in “upon him who listeth to obey”, Oliver Cowdery’s almost immediate correction in the printer’s manuscript.