Coarsely chop oats in a food processor and transfer to a large bowl.
Pulse together flour, butter, brown sugar, baking powder, and salt in processor until mixture resembles coarse meal.
Add milk and blend until mixture just forms a dough.
Add to oats and knead until just incorporated.
Halve dough and pat each half into a 5- by 3-inch rectangle.
Chill, wrapped well in plastic wrap, until firm, about 3 hours.
Preheat oven to 350F.
Cut 1 rectangle crosswise into scant 1/4-inch-thick slices and bake on an ungreased large baking sheet in middle of oven until undersides are a shade darker, about 20 minutes.
Transfer to a rack to cool and make more biscuits with remaining dough.