diff --git a/Packages/nadena.dev.modular-avatar/Editor/AvatarProcessor.cs b/Packages/nadena.dev.modular-avatar/Editor/AvatarProcessor.cs
index efba2b74..dc1761ba 100644
--- a/Packages/nadena.dev.modular-avatar/Editor/AvatarProcessor.cs
+++ b/Packages/nadena.dev.modular-avatar/Editor/AvatarProcessor.cs
@@ -22,6 +22,7 @@
  * SOFTWARE.
  */
 
+using System;
 using System.Runtime.CompilerServices;
 using UnityEditor;
 using UnityEngine;
@@ -60,5 +61,14 @@ namespace nadena.dev.modular_avatar.core.editor
         {
             ndmf.AvatarProcessor.ProcessAvatar(avatarGameObject);
         }
+
+        // ReSharper disable once InconsistentNaming
+        // ReSharper disable once MemberCanBeMadeStatic.Global
+        [Obsolete("This is only for compile time compatibility with legacy AAO")]
+        public int callbackOrder => throw new NotImplementedException();
+
+        [Obsolete("This is only for compile time compatibility with legacy AAO")]
+        // ReSharper disable once MemberCanBeMadeStatic.Global
+        public bool OnPreprocessAvatar(GameObject avatarGameObject) => throw new NotImplementedException();
     }
 }
\ No newline at end of file
diff --git a/Packages/nadena.dev.modular-avatar/Editor/LegacyAvatarOptimizerWarning.cs b/Packages/nadena.dev.modular-avatar/Editor/LegacyAvatarOptimizerWarning.cs
new file mode 100644
index 00000000..e87aee84
--- /dev/null
+++ b/Packages/nadena.dev.modular-avatar/Editor/LegacyAvatarOptimizerWarning.cs
@@ -0,0 +1,46 @@
+#if LEGACY_AVATAR_OPTIMIZER
+using UnityEditor;
+
+namespace nadena.dev.modular_avatar.core.editor
+{
+    [InitializeOnLoad]
+    public class LegacyAvatarOptimizerWarning
+    {
+        static LegacyAvatarOptimizerWarning()
+        {
+            if (SessionState.GetBool("nadena.dev.legacy-aao-warning", false)) return;
+            SessionState.SetBool("nadena.dev.legacy-aao-warning", true);
+            EditorApplication.delayCall += DisplayWarning;
+        }
+
+        private static void DisplayWarning()
+        {
+            var isJapanese = true;
+
+            while (true)
+            {
+                string message, readInOtherLang;
+                if (isJapanese)
+                {
+                    message = "1.4.x以前のAvatar Optimizerがインストールされているようです。\n" +
+                              "現在お使いのModular Avatarでは、1.4.x以前のAvatar Optimizerと互換性がありません。\n" +
+                              "Avatar Optimizerを1.5.0以降に更新してください！";
+                    readInOtherLang = "Read in English";
+                }
+                else
+                {
+                    message = "We found Avatar Optimizer 1.4.x or older is installed!\n" +
+                              "This version of Modular Avatar is not compatible with Avatar Optimizer 1.4.x or older!\n" +
+                              "Please upgrade Avatar Optimizer to 1.5.0 or later!";
+                    readInOtherLang = "日本語で読む";
+                }
+            
+                if (EditorUtility.DisplayDialog("Modular Avatar", message, "OK", readInOtherLang))
+                    return;
+
+                isJapanese = !isJapanese;
+            }
+        }
+    }
+}
+#endif
diff --git a/Packages/nadena.dev.modular-avatar/Editor/LegacyAvatarOptimizerWarning.cs.meta b/Packages/nadena.dev.modular-avatar/Editor/LegacyAvatarOptimizerWarning.cs.meta
new file mode 100644
index 00000000..85b0f31c
--- /dev/null
+++ b/Packages/nadena.dev.modular-avatar/Editor/LegacyAvatarOptimizerWarning.cs.meta
@@ -0,0 +1,3 @@
+fileFormatVersion: 2
+guid: 6c944e4e971248d59198fa569a47569f
+timeCreated: 1695800488
\ No newline at end of file
diff --git a/Packages/nadena.dev.modular-avatar/Editor/nadena.dev.modular-avatar.core.editor.asmdef b/Packages/nadena.dev.modular-avatar/Editor/nadena.dev.modular-avatar.core.editor.asmdef
index e81f1ca9..24bfa43c 100644
--- a/Packages/nadena.dev.modular-avatar/Editor/nadena.dev.modular-avatar.core.editor.asmdef
+++ b/Packages/nadena.dev.modular-avatar/Editor/nadena.dev.modular-avatar.core.editor.asmdef
@@ -29,6 +29,11 @@
     "autoReferenced": false,
     "defineConstraints": [],
     "versionDefines": [
+        {
+            "name": "com.anatawa12.avatar-optimizer",
+            "expression": "(,1.5.0-rc.8)",
+            "define": "LEGACY_AVATAR_OPTIMIZER"
+        }
     ],
     "noEngineReferences": false
 }
\ No newline at end of file