PREHEAT a large skillet or grill pan over medium-high heat.
RUB the chicken breasts with olive oil and season with salt and pepper.
COOK the chicken for about 6-8 minutes per side (depending on thickness), then remove from heat and allow to cool slightly. Dice when cool enough to handle.
IN a large bowl, mash the avocado with the mayonnaise, lime juice, onion, cilantro, and seasonings.
FOLD in the diced celery, bell pepper, and chicken.