Preheat oven to 350 degrees F (175 degrees C).

Mix rhubarb, white sugar, tapioca, and salt in a large bowl and toss to coat; let rest, stirring occasionally, for 30 minutes.

Stir strawberries into the rhubarb mixture; spread into a casserole dish.

Stir oats, brown sugar, flour, and butter together in a separate bowl until crumbly; sprinkle over the fruit mixture in the casserole dish.

Bake in preheated oven until set, about 45 minutes.