Mix brown sugar, flour, oats, butter, and cinnamon together in a bowl until crumbly. Press half the mixture into the bottom of an 8x8-inch pan; top with cherries.
Combine water, white sugar, cornstarch, and vanilla extract in a saucepan; bring to a simmer over medium heat. Cook and stir until sugar is dissolved and syrup is clear, about 5 minutes.
Pour syrup over cherries; top with remaining brown sugar-oat mixture.
Bake in the preheated oven until topping is browned and cherry filling is bubbling, 30 to 35 minutes.
