In a medium skillet, heat 1 tablespoon of the olive oil over medium heat.
Add onion and saute for about 3 minutes.
Add black beans with liquid from can, chili powder, and oregano. Reduce heat and simmer for approximately 5 minutes. When finished, season to taste with salt and pepper.
While bean mixture is simmering, in a medium saucepan heat 2 tablespoons of olive oil over medium heat. Place tortillas, one at a time in the hot oil and saute for about 30 seconds on each side. Repeat, adding and heating 2 tablespoons of oil to saute each tortilla.
On paper towels, drain each tortilla well.
Fill each tortilla with some black bean filling, shredded cheese, tomato, and avocado. Fold in half to make a taco.