Here Oliver Cowdery apparently missed the pronoun ye when he initially wrote this passage, but then almost immediately he added it (the level of ink flow is unchanged). The more common expression is without the ye:
(It should be noted that the first two of these are from Isaiah quotations.) There is one other occurrence of “O ye my people”:
Oliver’s correction in Mosiah 7:18 is not due to editing since the ye is not particularly expected.
Summary: Maintain Oliver Cowdery’s ye in Mosiah 7:18, the corrected reading in 𝓟 and most probably the original reading in 𝓞.