Place chicken in a 6-quart slow cooker; sprinkle evenly with salt and pepper. Add salsa and oregano. Cover and cook on HIGH until a thermometer inserted in thickest portion of thighs registers 165°F, about 2 hours. Turn off slow cooker. Transfer chicken to a large heatproof bowl; let cool 10 minutes. Shred meat into bite-size pieces. Add liquid in slow cooker to chicken; toss to coat. Serve over hot rice; garnish with cilantro, and serve with lime wedges.