diff --git a/Marlin/src/core/macros.h b/Marlin/src/core/macros.h
index 31808586cf..8e52152c14 100644
--- a/Marlin/src/core/macros.h
+++ b/Marlin/src/core/macros.h
@@ -70,11 +70,11 @@
 #define  FORCE_INLINE  __attribute__((always_inline)) inline
 #define NO_INLINE      __attribute__((noinline))
 #define _UNUSED      __attribute__((unused))
-#define _O0          __attribute__((optimize("O0")))
-#define _Os          __attribute__((optimize("Os")))
-#define _O1          __attribute__((optimize("O1")))
-#define _O2          __attribute__((optimize("O2")))
-#define _O3          __attribute__((optimize("O3")))
+#define __O0         __attribute__((optimize("O0")))
+#define __Os         __attribute__((optimize("Os")))
+#define __O1         __attribute__((optimize("O1")))
+#define __O2         __attribute__((optimize("O2")))
+#define __O3         __attribute__((optimize("O3")))
 
 #define IS_CONSTEXPR(...) __builtin_constant_p(__VA_ARGS__) // Only valid solution with C++14. Should use std::is_constant_evaluated() in C++20 instead
 
diff --git a/Marlin/src/feature/bedlevel/ubl/ubl.h b/Marlin/src/feature/bedlevel/ubl/ubl.h
index f117c1af65..8dad966acb 100644
--- a/Marlin/src/feature/bedlevel/ubl/ubl.h
+++ b/Marlin/src/feature/bedlevel/ubl/ubl.h
@@ -70,13 +70,13 @@ private:
     static void move_z_with_encoder(const_float_t multiplier);
     static float measure_point_with_encoder();
     static float measure_business_card_thickness();
-    static void manually_probe_remaining_mesh(const xy_pos_t&, const_float_t , const_float_t , const bool) _O0;
-    static void fine_tune_mesh(const xy_pos_t &pos, const bool do_ubl_mesh_map) _O0;
+    static void manually_probe_remaining_mesh(const xy_pos_t&, const_float_t , const_float_t , const bool) __O0;
+    static void fine_tune_mesh(const xy_pos_t &pos, const bool do_ubl_mesh_map) __O0;
   #endif
 
-  static bool G29_parse_parameters() _O0;
+  static bool G29_parse_parameters() __O0;
   static void shift_mesh_height();
-  static void probe_entire_mesh(const xy_pos_t &near, const bool do_ubl_mesh_map, const bool stow_probe, const bool do_furthest) _O0;
+  static void probe_entire_mesh(const xy_pos_t &near, const bool do_ubl_mesh_map, const bool stow_probe, const bool do_furthest) __O0;
   static void tilt_mesh_based_on_3pts(const_float_t z1, const_float_t z2, const_float_t z3);
   static void tilt_mesh_based_on_probed_grid(const bool do_ubl_mesh_map);
   static bool smart_fill_one(const uint8_t x, const uint8_t y, const int8_t xdir, const int8_t ydir);
@@ -98,17 +98,17 @@ public:
   static void report_state();
   static void save_ubl_active_state_and_disable();
   static void restore_ubl_active_state_and_leave();
-  static void display_map(const uint8_t) _O0;
-  static mesh_index_pair find_closest_mesh_point_of_type(const MeshPointType, const xy_pos_t&, const bool=false, MeshFlags *done_flags=nullptr) _O0;
-  static mesh_index_pair find_furthest_invalid_mesh_point() _O0;
+  static void display_map(const uint8_t) __O0;
+  static mesh_index_pair find_closest_mesh_point_of_type(const MeshPointType, const xy_pos_t&, const bool=false, MeshFlags *done_flags=nullptr) __O0;
+  static mesh_index_pair find_furthest_invalid_mesh_point() __O0;
   static void reset();
   static void invalidate();
   static void set_all_mesh_points_to_value(const_float_t value);
   static void adjust_mesh_to_mean(const bool cflag, const_float_t value);
   static bool sanity_check();
 
-  static void G29() _O0;                          // O0 for no optimization
-  static void smart_fill_wlsf(const_float_t ) _O2; // O2 gives smaller code than Os on A2560
+  static void G29() __O0;                           // O0 for no optimization
+  static void smart_fill_wlsf(const_float_t ) __O2; // O2 gives smaller code than Os on A2560
 
   static int8_t storage_slot;
 
diff --git a/Marlin/src/feature/bedlevel/ubl/ubl_motion.cpp b/Marlin/src/feature/bedlevel/ubl/ubl_motion.cpp
index f7e98c9fa7..3da4ed9809 100644
--- a/Marlin/src/feature/bedlevel/ubl/ubl_motion.cpp
+++ b/Marlin/src/feature/bedlevel/ubl/ubl_motion.cpp
@@ -340,7 +340,7 @@
    * Returns true if did NOT move, false if moved (requires current_position update).
    */
 
-  bool _O2 unified_bed_leveling::line_to_destination_segmented(const_feedRate_t scaled_fr_mm_s) {
+  bool __O2 unified_bed_leveling::line_to_destination_segmented(const_feedRate_t scaled_fr_mm_s) {
 
     if (!position_is_reachable(destination))  // fail if moving outside reachable boundary
       return true;                            // did not move, so current_position still accurate
diff --git a/Marlin/src/libs/bresenham.h b/Marlin/src/libs/bresenham.h
index 865c43c29e..39ab607007 100644
--- a/Marlin/src/libs/bresenham.h
+++ b/Marlin/src/libs/bresenham.h
@@ -30,7 +30,7 @@
  */
 
 #define FORCE_INLINE  __attribute__((always_inline)) inline
-#define _O3           __attribute__((optimize("O3")))
+#define __O3          __attribute__((optimize("O3")))
 
 template <uint8_t uid, uint8_t size>
 struct BresenhamCfg { static constexpr uint8_t UID = uid, SIZE = size; };
@@ -114,9 +114,9 @@ public:
     if (tick1(index)) { value[index] += dir[index]; back(index); }
   }
 
-  FORCE_INLINE static void tick1() _O3 { for (uint8_t i = 0; i < Cfg::SIZE; i++) (void)tick1(i); }
+  FORCE_INLINE static void tick1() __O3 { for (uint8_t i = 0; i < Cfg::SIZE; i++) (void)tick1(i); }
 
-  FORCE_INLINE static void tick() _O3 { for (uint8_t i = 0; i < Cfg::SIZE; i++) (void)tick(i); }
+  FORCE_INLINE static void tick() __O3 { for (uint8_t i = 0; i < Cfg::SIZE; i++) (void)tick(i); }
 
   static void report(const uint8_t index) {
     if (index < Cfg::SIZE) {
diff --git a/Marlin/src/libs/nozzle.h b/Marlin/src/libs/nozzle.h
index 7bbd0e35c1..69790f5a67 100644
--- a/Marlin/src/libs/nozzle.h
+++ b/Marlin/src/libs/nozzle.h
@@ -41,7 +41,7 @@ class Nozzle {
      * @param end xyz_pos_t defining the ending point
      * @param strokes number of strokes to execute
      */
-    static void stroke(const xyz_pos_t &start, const xyz_pos_t &end, const uint8_t &strokes) _Os;
+    static void stroke(const xyz_pos_t &start, const xyz_pos_t &end, const uint8_t &strokes) __Os;
 
     /**
      * @brief Zig-zag clean pattern
@@ -52,7 +52,7 @@ class Nozzle {
      * @param strokes number of strokes to execute
      * @param objects number of objects to create
      */
-    static void zigzag(const xyz_pos_t &start, const xyz_pos_t &end, const uint8_t &strokes, const uint8_t &objects) _Os;
+    static void zigzag(const xyz_pos_t &start, const xyz_pos_t &end, const uint8_t &strokes, const uint8_t &objects) __Os;
 
     /**
      * @brief Circular clean pattern
@@ -62,7 +62,7 @@ class Nozzle {
      * @param strokes number of strokes to execute
      * @param radius radius of circle
      */
-    static void circle(const xyz_pos_t &start, const xyz_pos_t &middle, const uint8_t &strokes, const_float_t radius) _Os;
+    static void circle(const xyz_pos_t &start, const xyz_pos_t &middle, const uint8_t &strokes, const_float_t radius) __Os;
 
   #endif // NOZZLE_CLEAN_FEATURE
 
@@ -77,14 +77,14 @@ class Nozzle {
      * @param pattern one of the available patterns
      * @param argument depends on the cleaning pattern
      */
-    static void clean(const uint8_t &pattern, const uint8_t &strokes, const_float_t radius, const uint8_t &objects, const uint8_t cleans) _Os;
+    static void clean(const uint8_t &pattern, const uint8_t &strokes, const_float_t radius, const uint8_t &objects, const uint8_t cleans) __Os;
 
   #endif // NOZZLE_CLEAN_FEATURE
 
   #if ENABLED(NOZZLE_PARK_FEATURE)
 
-    static float park_mode_0_height(const_float_t park_z) _Os;
-    static void park(const uint8_t z_action, const xyz_pos_t &park=NOZZLE_PARK_POINT) _Os;
+    static float park_mode_0_height(const_float_t park_z) __Os;
+    static void park(const uint8_t z_action, const xyz_pos_t &park=NOZZLE_PARK_POINT) __Os;
 
   #endif
 };
diff --git a/Marlin/src/module/endstops.cpp b/Marlin/src/module/endstops.cpp
index 3dd6d8aeb6..180f455734 100644
--- a/Marlin/src/module/endstops.cpp
+++ b/Marlin/src/module/endstops.cpp
@@ -494,7 +494,7 @@ static void print_es_state(const bool is_hit, FSTR_P const flabel=nullptr) {
 
 #pragma GCC diagnostic pop
 
-void _O2 Endstops::report_states() {
+void __O2 Endstops::report_states() {
   TERN_(BLTOUCH, bltouch._set_SW_mode());
   SERIAL_ECHOLNPGM(STR_M119_REPORT);
   #define ES_REPORT(S) print_es_state(READ(S##_PIN) != S##_ENDSTOP_INVERTING, F(STR_##S))
diff --git a/Marlin/src/module/temperature.h b/Marlin/src/module/temperature.h
index a4681ba02b..d1552d8f48 100644
--- a/Marlin/src/module/temperature.h
+++ b/Marlin/src/module/temperature.h
@@ -677,7 +677,7 @@ class Temperature {
     /**
      * Call periodically to manage heaters
      */
-    static void manage_heater() _O2; // Added _O2 to work around a compiler error
+    static void manage_heater() __O2; // __O2 added to work around a compiler error
 
     /**
      * Preheating hotends