Here Oliver Cowdery initially started to write “and now remember” in 𝓟. He had already written & now re when he stopped, erased the re, supralinearly inserted the exclamative O, and then continued by writing man inline. In other words, if Oliver hadn’t caught his error, he would have skipped the vocative O man. The fact that the re was erased and that man was written inline shows that the correction was immediate. Undoubtedly, the original manuscript read “and now O man remember and perish not”.
The vocative O man is used once more by king Benjamin in Mosiah 4:18 (“but I say unto you O man : whosoever doeth this the same hath great cause to repent”) as well as once by Nephi in 1 Nephi 10:20 (“therefore remember O man : for all thy doings thou shalt be brought into judgment”).
Summary: Accept Oliver Cowdery’s immediate correction that restored the vocative expression O man in Mosiah 4:30.