Cut the shrimp in half lengthwise, then into 3-4 small pieces.
Blend lime juice, lemon juice, and one serrano pepper (seeds removed) until smooth.
Place shrimp in a large bowl and pour the blended citrus mixture over it.
Add red onion, cilantro, diced serrano pepper, cucumber, oregano, salt, and pepper.
Gently toss everything together until well combined.
Cover and refrigerate for at least an hour to let the shrimp cook in the citrus juice.
Before serving, mix in the avocado.
Taste and adjust salt and pepper if needed. Enjoy!