Preheat grill to high (if using a charcoal grill, coals are ready when you can hold your hand 5 inches above grate for just 2 seconds). Lightly brush corn with oil, then season with salt and pepper. Grill corn, turning, until tender and blackened in places, about 8 minutes. Let cool slightly. Stand a cob on end, and carefully cut off kernels with a sharp knife using a downward motion. Repeat with other ear.
Put vinegar in a small bowl, and add oil in a slow, steady stream, whisking constantly, until emulsified. Combine spinach, onion, and grilled corn in a serving bowl; season with salt and pepper. Toss salad with dressing just before serving.