At first I got a little annoyed when I saw that O-lin came back in this chapter, but the more that I thought about it, the more I realize this is a good thing. Big Mom never got "Amnesia" at the beginning of this arc. O-Lin is a part of her personality disorder, which I've been saying she has since WCI. The clues were there cause she would always flip flop back and forth between a childlike mentality when she wanted to have fun and an enraged psychopath when she didn't get her way, and the two were constantly showing up depending on Big Mom's mood swings.
Happy to see I hit the nail on the head with this one. She's just like Cavendish and Hakuba, but in his case sleeping triggers his other personality to come out. We need more information, but with Big Mom it seems the prospect of dying is what triggers O-lin to take over, because we know extreme hunger triggers the psychopath.