Preheat oven to 350. Liberally spray a 10x13 cake pan with baking spray.
Combine the flour and sugar and stir until fully combined. Spread the berries in the cake pan, and then pour sugar mixture over the top. Stir gently to evenly coat. Some of the sugar may sit on the bottom of the pan. This is okay! Sprinkle lemon juice evenly over the berries.
Place the oatmeal, flour, butter, and brown sugar into a mixing bowl. Mix until the butter is in pea sized chunks and the mixture is crumbly.
Pour oatmeal mixture over the berries, and spread to evenly cover.
Bake at 350 for approximately one hour. Crisp is done when the berries are bubbly, the mixture has thickened, and the topping is lightly browned. Watch carefully! Baking times will vary depending on your specific oven, and how frozen your berries are when you bake. My berries were VERY frozen when I started mine.