How to Make It
Preheat oven to 400°F. Lightly coat an 8-inch square glass baking dish with cooking spray. Stir together strawberries, rhubarb, 1/2 cup of the brown sugar, and 2 tablespoons of the butter in a saucepan; cook over medium-high, stirring occasionally, until sugar is dissolved, fruits soften, and mixture begins to thicken, 8 to 9 minutes. Remove from heat.
Melt remaining 4 tablespoons butter in a microwavable bowl on HIGH until melted, about 45 seconds. Stir together melted butter, oats, flour, oil, salt, cinnamon, and remaining 1/4 cup brown sugar in a bowl until crumbly.
Pour strawberry mixture into prepared baking dish; sprinkle evenly with butter-oat mixture. Bake in preheated oven until topping is browned and fruit filling is bubbly, 15 to 18 minutes. Remove from oven; let cool 10 minutes.