Clean grill and prepare for direct cooking over medium heat, about 350 degrees F.
Brush corn with 2 Tbsp olive oil and season with 1/2 tsp salt. Grill over direct medium heat, with lid closed, until brown spots have formed and corn is tender. This will take 10-15 minutes. Turn corn occasionally. Remove from grill and cool enough to handle. Cut the kernels from the cobs and place into large bowl.
In a separate bowl whisk together the remaining olive oil and salt, lime juice, cumin, and pepper. Set dressing aside.
Add avocado, tomatoes, onion and cilantro to bowl with corn. Pour dressing on top. Toss to gently combine. Serve immediately.