Add blackberries, sugar, and lemon juice to a large saucepan and bring to a boil, stirring frequently. Once boiling, reduce the heat and simmer for 20-25 minutes, stirring occasionally.
Remove from heat and stir in the vanilla extract. Set aside.
Preheat the oven to 425° and grease a 10-inch cast iron skillet or 9x9 baking dish.
In a medium bowl, stir together rolled oats, brown sugar, flour, cinnamon, and salt until well combined. Add the cubed butter, working it in with a pastry cutter or fork until crumbly and well combined. Set aside.
Pour the blackberry filling into the baking dish, then top with the oat crisp topping.
Bake in the preheated oven for 10-15 minutes, until the filling is bubbly and the topping is browned and crispy.
Allow to cool at room temperature for about 30 minutes, then serve with ice cream!