Prepare grill for direct grilling; preheat to medium-high.
Brush peach halves lightly with oil, and place cut sides down on hot grill. Do not disturb for the first five minutes or so, as they will initially stick to the grill. When the cut sides have charred grill marks, gently turn the peaches and grill for 2 to 3 minutes more. Remove from heat.
for the streusel
While the peaches grill, add all of the streusel ingredients to a cast iron (or other grill-appropriate pan), stir to mix and place the pan on the grill. As the butter begins to melt, stir the ingredients together until the butter and sugar is melted and coats the oats and flour. Grill until the oats turn golden, then remove from heat.
to serve
Divide the streusel over four dessert plates. Place a peach half on top, and add one square of salted butter to the center. Drizzle with honey, and top with ice cream, if using.