This guy is the biggest big shot in town. He's the head of the House of Hwang and the richest man around. He may have been a respectable man at first, but over time, he and his family drive the great house into the ground.
The Old Lord's vice is lust. Oh, right—not just lust, but incest and pedophilia, too. He starts chasing after girls who haven’t even become preteens yet, girls who are technically his own daughters. Sounds like a charmer.
After a few years of unbridled lust and unbridled spending, the great house goes broke. The Old Lord himself has changed a lot in the process: "[T]he Old Lord, who had been fat, was now lean, and his skin hung in folds about him and he was unwashed and unshaven and his hand was yellow and trembled as he passed it over his chin and pulled at his loose old lips" (16.47). He dies shortly after this scene, right when Wang Lung buys up his land.
We see the Old Mistress more than we see the Old Lord because she was O-lan's owner. Her vice is opium. She sells all of the land in order to feed her addiction. She doesn’t care about anything, or anyone else. She just wants that opium.
Opium, no surprise, is also what kills her: “And when I came out they were gone and the Old Mistress sat dead in her chair, not from any touch they had given her but from fright. Her body was a rotten reed with the opium she smoked and she could not endure the fright" (16.58). It destroyed her from the inside out—which is a pretty good metaphor for what happens when lose your connection with the land.
Although the Old Lord and the Old Mistress are fairly unimportant characters, their legacy is pretty important. As Wang Lung's family gets richer and richer, they start looking more and more like the House of Hwang. Surprisingly, every time someone brings this up, people are flattered instead of worried.
When O-lan says that the kind of melancholy she sees in her first son is just like the kind of melancholy she saw in the nasty Young Lords in the great house, Wang Lung is proud (22.42). That makes sense, since he's kind of obsessed with the House of Hwang, which is to him "forever the great house" (28.45). At this point, Wang Lung isn't interested in the morality of the House of Hwang: he just wants to be like rich people, even if those people are despicable.
The great house itself almost seems cursed. As soon as Wang Lung's family moves in, they start to turn into mirror images of the people that they replaced. Wang Lung isn't "Wang The Farmer" anymore but "Wang The Big Man or Wang The Rich Man" (30.103). He becomes the Old Lord, chasing after young girls like Pear Blossom even though he is an old man. Lotus, of course, becomes the Old Mistress, getting fat and spoiled as only rich ladies can.
Only one thing is missing: what about the Young Lords? The ones who sold the land to Wang Lung in the first place? Oh, yeah: Wang Lung's own sons have become just like them. It's only at the end of his life that Wang Lung realizes where all of this is going: "‘It is the end of a family—when they begin to sell the land,' he said brokenly. 'Out of the land we came and into it we must go—and if you will hold your land you can live—no one can rob you of land […] if you sell the land, it is the end'" (34.93).
Can't you just guess what will happen next? Some farmer will buy their land until they have none left, and that will be the end of the House of Wang.
All his life, Wang Lung has wanted to be rich, but here's the reality: getting rich cuts you off from the land, and once you've been cut off from the land, you're done for.