Bring a large pot of salted water to a boil over high heat. Add garlic cloves, lemon halves, and chile. Add in shrimp and simmer until pink, about 5 minutes. Remove and rinse under cold water.
In a large bowl, toss shrimp, scallions, red onion, capers, celery, and tomatoes together.
In a small bowl, whisk together oregano, olive oil, salt, pepper, lemon juice, and vinegar. Pour over the salad and toss once more to incorporate. Cover and refrigerate until ready to serve.