I think the best heuristic for separating artist from their work is: How much does the artist try to put their own personality into it?
Woody Allen films are very much soaked through with Woody Allen. I don't get the sense that O'Brian's books are to the same degree.
I read Atlas Shrugged recently. I went in not knowing anything about the book or author. I liked it for maybe half way through and despised it by the end. Reading about the author after finishing the book, it had a lot of her personality in it.
You have to question what of the person filtered through to their works.
As a teen I voraciously read my way through the Belgariad, and later the Malloreon, Elenium and the Tamuli.
Shocked to learn that Eddings and his wife were both convicted and jailed for child abuse prior to writing those novels.