Preheat oven to 375°F / 190°C / Gas mark 5.
Make the filling - In a medium mixing bowl, combine cherries with flour, sugar, lemon juice, and zest, if using. Pour into a 1.5 quart baking dish.
Make topping - In a separate bowl, combine flour, oats, brown sugar, cinnamon and salt. Using a pastry blender, or your fingers, cut butter into flour/oat mixture until it resembles coarse crumbs.
Sprinkle topping on top of cherries and spread to cover evenly.
Bake for 40 - 45 minutes, until filling is bubbly and top is golden brown and crispy.
Cool 10 minutes before serving.