In a medium saucepan, boil 2 cups water. Add the rice, lower the heat to low, cover and cook until tender, about 15 minutes. Stir in half the lime juice.
In a small saucepan, mash half the beans with1/2 cup water over medium heat. Add the remaining beans and the oregano. Simmer for 8 minutes. Season with salt and pepper, turn off the heat and cover to keep warm.
Preheat a grill pan over medium-high heat. In a bowl, toss the onion, scallions and shrimp with the EVOO and chili powder; season. Grill the onions and scallion whites for 7 minutes, then the scallion greens and shrimp, turning once, for 4 minutes. Transfer to a plate.
In a small bowl, mash the avocado with the remaining lime juice. Divide the rice, beans, veggies and shrimp among 4 bowls and dollop with the avocado mash.