In a small pan bring water to a boil. Add beef bouillon cubes and stir until dissolved, remove from heat and set aside. In a large skillet brown the ground beef and onion over medium-high heat. Drain off any excess liquid.
Add oregano, cumin, garlic, chili powder, and pepper to the browned meat. Add beef bouillon/water mixture and tomato sauce and stir to combine. Simmer for 20 minutes or until thick.
While beef is simmering, split hamburger buns and butter each of the cut sides. Heat a large skillet to medium heat. Place split hamburger buns cut side down into the skillet and cook until lightly browned on the bottom.
Place taco meat on top of the bottom half of the hamburger bun, top with shredded cheese, lettuce, and tomato. Repeat with remaining sandwiches.