Slice the top off the tomatoes and scoop out the seeds using either a paring knife or a spoon.
Place the tomatoes in the oven, covered with foil. Bake 5–7 minutes to soften tomatoes slightly. Don’t bake too long or the tomatoes will become mushy and fall apart. Take out of the oven and dump out any water in bottom of tomatoes.
While tomatoes are baking, sauté the onion in oil for a few minutes until soft.
Add the bison and oregano, breaking the meat into small pieces as it cooks. Sauté five minutes, at which point the bison should only be slightly pink.
Season to taste.
Turn oven to broil.
Spoon meat into each tomato, pressing the meat down to fill the tomato as much as possible while still leaving about a 1/2 inch of space on top for the egg.
Crack an egg into each tomato and put under broiler until egg is just set, about 5 minutes.