For the topping: Preheat the oven to 375 degrees with rack in the middle position. In a large bowl, whisk together flour, oats, sugar, cinnamon, and salt. Stir in oil and water until blended. Topping will look like clumpy wet sand. Stir in almonds and set aside.
For the filling: In a large saucepan of boiling water, working in batches, blanch peaches for 1 minute. Remove them with a slotted spoon and transfer to a bowl of cold water. When cool enough to handle, peel off skin. Halve, pit, and cut peaches into 1/2-inch wedges.
In a medium bowl, mix together peaches, lemon juice, flour, and sugar. Place filling in an 8-inch square baking pan. Sprinkle topping evenly over peaches. Bake until the topping turns a deep, golden brown and the filling is bubbling around the edges, 30 to 35 minutes. Serve warm or at room temperature with vanilla ice cream, if desired.