Heat the oil on medium heat. Cook the onion and the bell peppers until the onion is transparent and fragrant, around 3-5 minutes. Transfer to a small bowl and set aside.
In a small bowl, mix together the ground bison, Worcestershire sauce, salt and pepper until fully combined. Form into 4 patties.
In the same skillet you used for the onions, cook the bison patties until no longer pink, around 5 minutes per side.
Spread 1 Tbsp of spicy ketchup on top of each bun. Top with one patty and some cooked onion mix.