Here scribe 2 of 𝓞 seems to have accidentally omitted the and. See 1 Nephi 4:10–11 and 1 Nephi 12:11 for examples of this tendency on his part. In most instances, scribe 2 of 𝓞 caught this type of error, but not in this instance. Here Oliver Cowdery later added the & supralinearly in 𝓞 with a sharper quill (but without any insert mark). Perhaps Oliver’s correction occurred as he was copying the text into 𝓟. This emendation was probably motivated by Oliver’s uneasiness about the text as scribe 2 of 𝓞 had written it. The word also seems to require an and. In fact, “and also” is very common in the text. A similar passage, also in this chapter, shows that “and also” is expected when conjoining “from the tree of life” with “from the righteous” (semantically equivalent to “from the saints of God”):
Summary: Accept Oliver Cowdery’s decision in 1 Nephi 15:28 to add and to the conjunct “also from the saints of God” that follows “from the tree of life”.