If cooking over a campfire, prepare a hot fire with a grate about 6 inches from the heat.
Set a Dutch oven on the grate over medium heat (or over medium heat on your stove) and add the oil.
Stir in the onions and sauté until softened, about 6 minutes. Add the garlic and cook, stirring constantly, for 1 minute.
Add the ground bison, season with salt and pepper, and cook until the meat is browned, 6 to 7 minutes.
Stir in the tomato paste, chile powder, cumin, and oregano, and cook until the tomato paste darkens and everything is fragrant, about 2 minutes.
Pour in the crushed tomatoes and broth and bring to a simmer.
Simmer the chili, partially covered, stirring occasionally, until very thick, about 1 hour. Stir in the beans during the last 10 to 15 minutes to allow them to heat through. Taste and adjust seasoning, if needed.
Serve warm, topped with sour cream, grated cheese, and chopped scallions, if desired.