1. Mix together 3 tablespoons of the olive oil, the garlic, lime juice, salt, oregano and pepper in a shallow glass baking dish. Add shrimp; toss the shrimp to coat evenly with the marinade. Cover the dish; refrigerate for 30 to 45 minutes.
2. Mix together the mayonnaise and salsa in a small bowl. Cover and refrigerate.
3. Heat the remaining tablespoon oil in a large nonstick skillet over medium-high heat. Remove the shrimp from the marinade with a slotted spoon, reserving the marinade. Add the shrimp to the skillet in an even layer; cook about 2 minutes on each side or just until the shrimp are cooked through.
4. Add the reserved marinade to the shrimp in the skillet. Bring to simmering over medium-high heat. Remove the skillet from heat. Stir in the rice until well blended. Cover the skillet; let stand for 10 minutes or until the rice is tender.
5. Meanwhile, gently warm the corn tortillas according to the package directions.
6. To serve the tacos, place about 1/3 cup of the shrimp and rice mixture on one half of each warm tortilla. Season with a squeeze of fresh lime juice, top with a scattering of shredded lettuce and spoon on a small amount of the salsa-mayonnaise mixture. Roll up the tortilla to make the taco. Makes 12 soft tacos.