Over a gas or charcoal grill, grill the corn on all sides until it gets a nice sear. Using a sharp knife, slice the corn kernels off the cob. Slice the tomatoes into halves, and dice the avocado and onion.
In a small bowl, whisk the oil, lime juice, and grated garlic together. Season the dressing to taste with salt and pepper. Place the chopped veggies into a serving bowl and drizzle them with the salad dressing.