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