In a large bowl combine the olive oil, lemon juice, onion, and salt and pepper.
Add the shrimp, avocado, tomatoes, corn and jalapeno, and toss to coat with the dressing. Transfer to a serving bowl. Garnish with the cilantro, if using.
Leftover Summer Shrimp and Avocado Salad turned into this pasta salad the next day. When I made it from scratch another time, I had the advantage of knowing that I could dice and add the avocado right before serving, which was the only real drawback of making it from leftovers.