Heat 1 tablespoon oil in a skillet over medium heat. Add onion and garlic and cook for 3- 4 minutes. Transfer to a large bowl and combine with venison. Season with salt and pepper.
In another bowl, combine ricotta and half of the mozzarella and Parmesan cheeses. Add Italian seasoning. Combine cheese mixture with meat mixture and stir to blend.
Cook manicotti in a large pot of boiling salted water for 4-6 minutes. Lightly coat a baking sheet with 1 tablespoon oil. When cooked, use a slotted spoon to transfer manicotti to the oiled baking sheet and allow to cool.
Coat a 13 X 9 X 2 inch baking dish with remaining 2 tablespoons oil. Spread half of the marinara sauce over the bottom of the pan. Spoon meat and cheese mixture into cooled manicotti tubes and arrange in a single layer in the dish. Spoon remaining sauce over and top with remaining cheeses.
Bake, uncovered, in a preheated 350 degree oven until heated throughout and bubbly, about 30 minutes. Let stand for 5 minutes before serving (this allows mixture to set up)