I keep wondering if her abrupt departure from the show, which is reported to have been her decision, was because people's hatred of her character spilled over into her personal life.
As I recall, the story had it that she left to be lady's maid to Rose's mother, that sour lady Susan, I think? But then later Susan visits during Rose's wedding days and doesn't have O'brien with her and that's never addressed.
Unless I'm remembering wrong, anyway. It's always bugged me that we didn't get some storyline on that.