Heat the olive oil in a large, deep skillet or pot over medium heat. Add the onions, cook approximately 3-4 minutes, stirring frequently.
Add the garlic and oregano and cook 1 minute, stirring frequently.
Add the sausage, raise the heat to medium-high and cook for 5 minutes while stirring and breaking up any large pieces with a wooden spoon or a spatula.
Add the pasta sauce, diced tomatoes and beef broth to the sausage and stir well. Bring the sauce to a simmer, lower heat to medium-low and cover. Cook the sauce for 30-35 minutes, stirring occasionally. Keep the sauce at a simmer. If it starts boiling too vigorously, lower the heat.
Taste the tomato sauce and add salt, if needed. As a reference we did not add extra salt to ours.
When the sauce is almost done, cook the spaghetti according to package directions for al dente. Remember to salt the water before dropping in the pasta.
Drain the spaghetti, serve it with the Italian sausage sauce and garnish with chopped parsley, if desired.