Mix oats, flour, brown sugar, and butter together in a bowl; press mixture into the bottom of a 9-inch pie pan. Top with strawberries and rhubarb.
Stir water, white sugar, and cornstarch together in a saucepan. Bring to simmer, constantly whisking, until mixture bubbles and thickens; pour over fruit layer.
Bake in the preheated oven until bubbling, about 1 hour.
