Mix oats and coconut in a large bowl until thoroughly combined.
Stir sugar, cocoa powder, milk, and butter together in a saucepan over medium heat until smooth. Bring to a boil and cook, stirring constantly, for 2 minutes. Pour over oats and coconut; mix quickly to coat.
Drop tablespoonfuls onto the prepared baking sheet. Let cookies cool and harden; store in an airtight container.