Heat oven to 400°F. Place the tortillas directly on middle oven rack. Bake until golden brown in spots and around edges, about 5 minutes. Transfer to a work surface; set aside.
In a large bowl, stir together cabbage, beans, sweet pepper, oregano, olive oil, salt and pepper. Just before serving, gently toss in the watermelon.
Spread avocado evenly on the tortillas. Drizzle with lemon juice from half of the lemon. Top with prepared watermelon slaw and remaining lemon juice.