Who is Luke O'Neil in The Thorn Birds by Colleen McCullough most similar to in The Kite Runner and A Thousand Splendid Suns by Khaled Hosseini?

That's an interesting set of comparisons to make since, at least prima facie, there are no obvious resemblances between The Thorn Birds and Hosseini's novels. One way to attempt to answer this is to first characterize Luke O'Neill and then consider the characters from A Thousand Splendid Suns and The Kite Runner.

Luke treats women as objects and marries Meggie for mercenary reasons. He's brutish as well as miserly and treats Meggie very poorly. He also seems not to care at all about his daughter, Justine.

In A Thousand Splendid Suns Rasheed is the character most similar to Luke. He is reprehensible and inflicts abuse upon his wives until one of them (Mariam) finally kills him. (Meggie leaves Luke, which is rather less drastic!) Rasheed is also disinterested in the daughter that his second wife (Laila) bears him.

In The Kite Runner, Assef is probably the closest to Luke in character. There are not too many parallels between their lives, but both characters are cruel and sadistic and have little regard for anyone besides themselves.