How did o lan die?

I don't think its known

She dies of natural causes.......

Natural causes can be pretty much anything (Cancer, old age...)For what it's worth,

While the novel does not specify exactly how o-lan dies, her symptoms along with the area and time period are consistent with that of suffering from an intestinal parasite.

One day around the time Wang Lung began to feel guilt for casting her aside, she can no longer leave her room. Wang Lung summons the town's doctor who says among other things, that she has worms in her heart. The doctor says then that it will cost 500 pieces of silver to help her, but, O-lan refuses, saying good land could be bought with that money. Wang Lung ignores her and exclaims that he has the money. Upon hearing this, the doctor says that the color of her eyes are different and treatment will cost 5,000 pieces of silver. Wang Lung realizes that her case is terminal, does not answer, and the doctor simply says, "She will die". Over the next years, O-lan becomes weaker and begins to fall into fits. She constantly shows how much Wang Lung has hurt her when she constantly murmurs at the end "I am ugly and cannot be loved". In the end, Wang Lung cannot bear to be in her presence. When O-lan finally died of what can be assumed is some kind of cancer after insisting seeing her eldest son married, she was buried with her father-in-law on Wang Lung's land in a very rich manner.

♥♠♦♣ IT SAYS THAT SHE DIES OF NATERAL CAUSES♣♦♠♥