diff --git a/lib/open_api_spex/operation_dsl.ex b/lib/open_api_spex/operation_dsl.ex
index e2c81dc7..3f36bae2 100644
--- a/lib/open_api_spex/operation_dsl.ex
+++ b/lib/open_api_spex/operation_dsl.ex
@@ -1,17 +1,27 @@
 defmodule OpenApiSpex.OperationDsl do
   alias OpenApiSpex.OperationBuilder
 
+  @doc """
+  Defines an Operation spec in a controller.
+  """
   defmacro operation(action, spec) do
     operation_def(action, spec)
   end
 
+  @doc """
+  Defines a list of tags that all operations in a controller will share.
+  """
   defmacro tags(tags) do
     tags_def(tags)
   end
 
+  @doc false
   defmacro before_compile(_env) do
     quote do
-      def spec_attributes, do: @spec_attributes
+      def open_api_operation(action) do
+        module_name = __MODULE__ |> to_string() |> String.replace_leading("Elixir.", "")
+        IO.warn("No operation spec defined for controller action #{module_name}.#{action}")
+      end
 
       def controller_tags do
         Module.get_attribute(__MODULE__, :controller_tags) || []
@@ -19,6 +29,7 @@ defmodule OpenApiSpex.OperationDsl do
     end
   end
 
+  @doc false
   def operation_def(action, spec) do
     quote do
       if !Module.get_attribute(__MODULE__, :operation_defined) do
@@ -30,15 +41,21 @@ defmodule OpenApiSpex.OperationDsl do
       end
 
       @spec_attributes {unquote(action), operation_spec(__MODULE__, unquote(spec))}
+
+      def open_api_operation(unquote(action)) do
+        @spec_attributes[unquote(action)]
+      end
     end
   end
 
+  @doc false
   def tags_def(tags) do
     quote do
       @controller_tags unquote(tags)
     end
   end
 
+  @doc false
   def operation_spec(module, spec) do
     spec = Map.new(spec)
     tags = spec[:tags] || Module.get_attribute(module, :controller_tags)
@@ -51,7 +68,8 @@ defmodule OpenApiSpex.OperationDsl do
 
     spec = Map.delete(spec, :parameters)
 
-    struct!(OpenApiSpex.Operation, initial_attrs)
+    OpenApiSpex.Operation
+    |> struct!(initial_attrs)
     |> struct!(spec)
   end
 end
diff --git a/test/operation_dsl_test.exs b/test/operation_dsl_test.exs
index 18422aec..d5c4f7c2 100644
--- a/test/operation_dsl_test.exs
+++ b/test/operation_dsl_test.exs
@@ -1,44 +1,56 @@
 defmodule OpenApiSpex.OperationDslTest do
   use ExUnit.Case, async: true
 
+  import ExUnit.CaptureIO
+
   alias OpenApiSpexTest.DslController
 
-  test "operation/1" do
-    assert [
-             show: %OpenApiSpex.Operation{
+  describe "operation/1" do
+    test "defines open_api_operation/1 for :show action" do
+      assert %OpenApiSpex.Operation{
                responses: [],
                summary: "Show user",
                parameters: show_parameters,
                tags: show_tags
-             },
-             index: %OpenApiSpex.Operation{
+             } = DslController.open_api_operation(:show)
+
+      assert show_tags == ["users"]
+
+      assert [
+               %OpenApiSpex.Parameter{
+                 description: "User ID",
+                 example: 1001,
+                 in: :path,
+                 name: :id,
+                 required: true,
+                 schema: %OpenApiSpex.Schema{type: :integer}
+               }
+             ] = show_parameters
+    end
+
+    test "defines open_api_operation/1 for :index action" do
+      assert %OpenApiSpex.Operation{
                responses: [],
                summary: "User index",
                parameters: index_parameters
-             }
-           ] = DslController.spec_attributes()
-
-    assert show_tags == ["users"]
-
-    assert [
-             %OpenApiSpex.Parameter{
-               description: "User ID",
-               example: 1001,
-               in: :path,
-               name: :id,
-               required: true,
-               schema: %OpenApiSpex.Schema{type: :integer}
-             }
-           ] = show_parameters
-
-    assert [
-             %OpenApiSpex.Parameter{
-               description: "Free-form query string",
-               example: "jane",
-               in: :query,
-               name: :query,
-               schema: %OpenApiSpex.Schema{type: :string}
-             }
-           ] = index_parameters
+             } = DslController.open_api_operation(:index)
+
+      assert [
+               %OpenApiSpex.Parameter{
+                 description: "Free-form query string",
+                 example: "jane",
+                 in: :query,
+                 name: :query,
+                 schema: %OpenApiSpex.Schema{type: :string}
+               }
+             ] = index_parameters
+    end
+
+    test "outputs warning when action not defined for called open_api_operation" do
+      output = capture_io(:stderr, fn -> DslController.open_api_operation(:undefined) end)
+
+      assert output =~
+               ~r/warning:.*No operation spec defined for controller action OpenApiSpexTest.DslController.undefined/
+    end
   end
 end