In the Game of Thrones episode the Gift, Oleanna threatens Lord 'Littlefinger' Baelish with revealing that he helped her poison Joffrey.
I can see Oleanna wanting to protect Margery from being married to the psychopath, but what did Baelish have to gain by helping her do it?
Okay, less cryptically, the Lannisters are starting to get a proper foothold on the Iron Throne - if Littlefinger wants to climb the ladder of power from the Vale to the Iron Throne,
as he states to Sansa later in the show for his goal to be there with Sansa as his queen,
then eventually he will need to confront the fact that the Lannisters would have at least one marriage settled with Joffrey and Margaery, potentially another with Tommen and someone else, and Mycrella with Oberyn's nephew, Cersei with Loras, and Tyrion with Sansa (Though at the time of planning the collaboration, the latter two weren't considered as I understand it yet? At least considering Cersei and Tyrion as possible "off branches" for the Lannister dynasty, if necessary.).
By helping with the poisoning, he breaks up the Joffrey and Margaery marriage, which does bounce into Tommen and Margaery, but that removes Tommen from having a separate marriage alliance, and the method of poisoning ends up implicating another (By happenstance, this starts to remove Tyrion from the board.), and the infighting breaks up their hold on King's Landing. This works almost too well, because by the time Littlefinger returns, the Faith Militant have risen to power, and Olenna is asking for his help in dealing with them. One could say that, post poisoning, he rose up the social ladder in King's Ladder simply by not being there, even if he got lucky by how destructive the Lannisters were after the Purple Wedding to each other.