The character who appears in both "The Stand" and "The Dark Tower" as the Man in Black is Randall Flagg.
Flagg is one of Stephen King’s recurring supernatural villains. In The Stand, he is the charismatic, demonic figure who gathers survivors in Las Vegas after a plague devastates most of the population. In The Dark Tower books, he becomes a major adversary of Roland Deschain and is tied to the wider mythology connecting many of King’s stories.
Walter O’Dim is not a different character: it is one of Flagg’s principal aliases in The Dark Tower. Other names include the Walking Dude, Marten Broadcloak, and Walter Padick. The Crimson King is also a powerful villain in the same fictional universe, but he is a separate entity whom Flagg serves.
Flagg’s appearances illustrate King’s interconnected fictional cosmos. Although details vary across books, his recurring identity is the dangerous Man in Black—not Maturin or the Crimson King.