Whisk together oil, vinegar, ¾ teaspoon salt, and ¼ teaspoon pepper. Toss tomatoes with dressing.
While tomatoes marinate, cook white parts of scallions in butter with ¾ teaspoon salt and ½ teaspoon pepper in a 12-inch heavy skillet over medium-high heat, stirring occasionally, until golden, about 4 minutes.
Add corn and sauté until just tender, about 5 minutes. Transfer to a bowl and cool.
Stir together corn, tomatoes, and scallion greens.