Cut the shrimp in half lengthwise (so it makes two thin slices of shrimp) and then cut each half into 3 to 4 smaller pieces, depending on how chunky you want your ceviche!
In a food processor, add the lime juice, lemon juice, and 1 serrano pepper. Blend until smooth.
Place the shrimp in a large bowl. Add the serrano-citrus mixture, red onion, cilantro, the remaining serrano pepper (seeds removed and diced small), cucumber, oregano, salt, and pepper. Gently toss to combine. Cover and refrigerate for at least 1 hour to let the flavors come together and for the shrimp to cook through.
Before serving, add the diced avocado and toss to combine. Taste and season with more salt and pepper, if needed. Serve as desired!