Soak onion in hot vinegar if you want them pickled. See the post above for details. You can put them in raw if you prefer.
Cook corn according to package directions or until tender if you're using fresh corn. If you want to roast the corn, you can grill it or put it in an iron skillet and heat until it's charred a little.
Dice avocado and squeeze a little lime juice over them to prevent them from turning brown.
Combine chicken, avocado, corn, red onion, green onion, cilantro, salt, pepper, lime juice, and olive oil. Gently toss to coat. Taste and adjust seasoning if necessary.
Chill or serve immediately. You can make this a few hours in advance, but I wouldn't make it more than 3 hours in advance. The avocado breaks down quickly.