Bring a large pot of salted water to a boil over high heat. Add the pasta and cook until tender but still firm to the bite, stirring occasionally, about 8 to 10 minutes. Drain pasta reserving 1 cup of the cooking liquid.
In a large, heavy skillet warm the olive oil over medium-high heat. Add the sausage and onions. Using a wooden spoon break up the sausage into bite-sized pieces as it browns.
Continue cooking until the sausage is golden and the onions are tender. Add the beans and oregano cook for 2 more minutes. Add the cup of pasta cooking liquid and stir, scrapping up any brown bits from the bottom of the pan. Add the mascarpone cheese and stir until it dissolved into a light sauce. Add the salt, pepper, and hot pasta. Stir until coated and serve.