diff --git a/ports/broadcom/Makefile b/ports/broadcom/Makefile
index 240362e3de7f6..f60ebbf045e1f 100644
--- a/ports/broadcom/Makefile
+++ b/ports/broadcom/Makefile
@@ -95,7 +95,7 @@ else
 endif
 
 
-CFLAGS += $(INC) -Wall -Werror -std=gnu11 $(BASE_CFLAGS) $(CFLAGS_MOD) $(COPT) $(DISABLE_WARNINGS)
+CFLAGS += $(INC) -Wall -Werror -std=gnu11 $(BASE_CFLAGS) $(CFLAGS_MOD) $(COPT) $(DISABLE_WARNINGS) -Werror=old-style-definition
 
 $(BUILD)/lib/tlsf/tlsf.o: CFLAGS += -Wno-cast-align
 
diff --git a/ports/cxd56/common-hal/camera/Camera.c b/ports/cxd56/common-hal/camera/Camera.c
index c3c935a168d42..b9fa2787ef767 100644
--- a/ports/cxd56/common-hal/camera/Camera.c
+++ b/ports/cxd56/common-hal/camera/Camera.c
@@ -44,7 +44,7 @@ static const image_size_t isx019_image_size_table[] = {
     { VIDEO_HSIZE_QUADVGA, VIDEO_VSIZE_QUADVGA },
 };
 
-static const char *get_imgsensor_name() {
+static const char *get_imgsensor_name(void) {
     static struct v4l2_capability cap;
 
     ioctl(camera_dev.fd, VIDIOC_QUERYCAP, (unsigned long)&cap);
@@ -113,7 +113,7 @@ static void camera_start_streaming(enum v4l2_buf_type type) {
     ioctl(camera_dev.fd, VIDIOC_STREAMON, (unsigned long)&type);
 }
 
-static void camera_start_preview() {
+static void camera_start_preview(void) {
     camera_set_format(V4L2_BUF_TYPE_VIDEO_CAPTURE, V4L2_PIX_FMT_UYVY, VIDEO_HSIZE_QVGA, VIDEO_VSIZE_QVGA);
 
     v4l2_buffer_t buf;
diff --git a/ports/espressif/Makefile b/ports/espressif/Makefile
index 542083188e556..09522ea795050 100644
--- a/ports/espressif/Makefile
+++ b/ports/espressif/Makefile
@@ -191,7 +191,7 @@ endif
 # option to override compiler optimization level, set in boards/$(BOARD)/mpconfigboard.mk
 CFLAGS += $(OPTIMIZATION_FLAGS)
 
-CFLAGS += $(INC) -Werror -Wall -std=gnu11 -Wl,--gc-sections $(BASE_CFLAGS) $(C_DEFS) $(CFLAGS_MOD) $(COPT) -Werror=missing-prototypes
+CFLAGS += $(INC) -Werror -Wall -std=gnu11 -Wl,--gc-sections $(BASE_CFLAGS) $(C_DEFS) $(CFLAGS_MOD) $(COPT) -Werror=missing-prototypes -Werror=old-style-definition
 
 # Most current ESPs have nano versions of newlib in ROM so we use them.
 ifneq ($(IDF_TARGET),esp32c6)
diff --git a/ports/espressif/common-hal/_bleio/__init__.c b/ports/espressif/common-hal/_bleio/__init__.c
index f3637a8fddb99..119fe55921e8a 100644
--- a/ports/espressif/common-hal/_bleio/__init__.c
+++ b/ports/espressif/common-hal/_bleio/__init__.c
@@ -31,7 +31,7 @@ static uint64_t _timeout_start_time;
 
 background_callback_t bleio_background_callback;
 
-void bleio_user_reset() {
+void bleio_user_reset(void) {
     // Stop any user scanning or advertising.
     common_hal_bleio_adapter_stop_scan(&common_hal_bleio_adapter_obj);
     common_hal_bleio_adapter_stop_advertising(&common_hal_bleio_adapter_obj);
@@ -43,7 +43,7 @@ void bleio_user_reset() {
 }
 
 // Turn off BLE on a reset or reload.
-void bleio_reset() {
+void bleio_reset(void) {
     // Set this explicitly to save data.
     if (!common_hal_bleio_adapter_get_enabled(&common_hal_bleio_adapter_obj)) {
         return;
diff --git a/ports/espressif/common-hal/sdioio/SDCard.c b/ports/espressif/common-hal/sdioio/SDCard.c
index 6fd27c289459c..87ba32f802095 100644
--- a/ports/espressif/common-hal/sdioio/SDCard.c
+++ b/ports/espressif/common-hal/sdioio/SDCard.c
@@ -245,7 +245,7 @@ void common_hal_sdioio_sdcard_never_reset(sdioio_sdcard_obj_t *self) {
     }
 }
 
-void sdioio_reset() {
+void sdioio_reset(void) {
     for (size_t i = 0; i < MP_ARRAY_SIZE(slot_in_use); i++) {
         if (!never_reset_sdio[i]) {
             slot_in_use[i] = false;
diff --git a/ports/espressif/supervisor/port.c b/ports/espressif/supervisor/port.c
index e72eab7e32a56..121335245dc33 100644
--- a/ports/espressif/supervisor/port.c
+++ b/ports/espressif/supervisor/port.c
@@ -461,11 +461,11 @@ void port_disable_tick(void) {
     esp_timer_stop(_tick_timer);
 }
 
-void port_wake_main_task() {
+void port_wake_main_task(void) {
     xTaskNotifyGive(circuitpython_task);
 }
 
-void port_wake_main_task_from_isr() {
+void port_wake_main_task_from_isr(void) {
     BaseType_t xHigherPriorityTaskWoken = pdFALSE;
     vTaskNotifyGiveFromISR(circuitpython_task, &xHigherPriorityTaskWoken);
     if (xHigherPriorityTaskWoken == pdTRUE) {
@@ -473,7 +473,7 @@ void port_wake_main_task_from_isr() {
     }
 }
 
-void port_yield() {
+void port_yield(void) {
     vTaskDelay(4);
 }
 
diff --git a/ports/litex/Makefile b/ports/litex/Makefile
index bb1d88e755201..98abe985699cf 100644
--- a/ports/litex/Makefile
+++ b/ports/litex/Makefile
@@ -38,7 +38,7 @@ endif
 # option to override compiler optimization level, set in boards/$(BOARD)/mpconfigboard.mk
 CFLAGS += $(OPTIMIZATION_FLAGS)
 
-CFLAGS += $(INC) -Werror -Wall -std=gnu11 -nostdlib -fshort-enums $(BASE_CFLAGS) $(C_DEFS) $(CFLAGS_MOD) $(COPT) -Werror=missing-prototypes
+CFLAGS += $(INC) -Werror -Wall -std=gnu11 -nostdlib -fshort-enums $(BASE_CFLAGS) $(C_DEFS) $(CFLAGS_MOD) $(COPT) -Werror=missing-prototypes -Werror=old-style-definition
 
 # TODO: check this
 CFLAGS += -D__START=main -DFOMU
diff --git a/ports/mimxrt10xx/Makefile b/ports/mimxrt10xx/Makefile
index 57a8beb069129..e6928eead619d 100644
--- a/ports/mimxrt10xx/Makefile
+++ b/ports/mimxrt10xx/Makefile
@@ -45,7 +45,7 @@ ifeq ($(DEBUG), 1)
   CFLAGS += -fno-ipa-sra
 endif
 
-CFLAGS += $(INC) -ggdb -Wall -Werror -std=gnu11 -nostdlib -fshort-enums $(BASE_CFLAGS) $(CFLAGS_MOD) $(COPT) -Werror=missing-prototypes
+CFLAGS += $(INC) -ggdb -Wall -Werror -std=gnu11 -nostdlib -fshort-enums $(BASE_CFLAGS) $(CFLAGS_MOD) $(COPT) -Werror=missing-prototypes -Werror=old-style-definition
 
 # TODO: add these when -Werror is applied
 # Disable some warnings, as do most ports. NXP SDK causes undef, tinyusb causes cast-align
diff --git a/ports/mimxrt10xx/common-hal/audiobusio/__init__.c b/ports/mimxrt10xx/common-hal/audiobusio/__init__.c
index 196e52dbf0369..f1030bfcd848c 100644
--- a/ports/mimxrt10xx/common-hal/audiobusio/__init__.c
+++ b/ports/mimxrt10xx/common-hal/audiobusio/__init__.c
@@ -439,6 +439,6 @@ void port_i2s_resume(i2s_t *self) {
     self->paused = false;
 }
 
-void i2s_reset() {
+void i2s_reset(void) {
 // this port relies on object finalizers for reset
 }
diff --git a/ports/nordic/Makefile b/ports/nordic/Makefile
index e60b4072a8d48..aa614e097ba3c 100755
--- a/ports/nordic/Makefile
+++ b/ports/nordic/Makefile
@@ -47,7 +47,7 @@ endif
 # option to override compiler optimization level, set in boards/$(BOARD)/mpconfigboard.mk
 CFLAGS += $(OPTIMIZATION_FLAGS)
 
-CFLAGS += $(INC) -Wall -Werror -std=gnu11 -nostdlib -fshort-enums $(BASE_CFLAGS) $(CFLAGS_MOD) $(COPT) -Werror=missing-prototypes
+CFLAGS += $(INC) -Wall -Werror -std=gnu11 -nostdlib -fshort-enums $(BASE_CFLAGS) $(CFLAGS_MOD) $(COPT) -Werror=missing-prototypes -Werror=old-style-definition
 
 # Nordic Softdevice SDK header files contains inline assembler that has
 # broken constraints. As a result the IPA-modref pass, introduced in gcc-11,
diff --git a/ports/nordic/common-hal/_bleio/__init__.c b/ports/nordic/common-hal/_bleio/__init__.c
index 095723194cfbf..454937dcd354a 100644
--- a/ports/nordic/common-hal/_bleio/__init__.c
+++ b/ports/nordic/common-hal/_bleio/__init__.c
@@ -80,7 +80,7 @@ void check_sec_status(uint8_t sec_status) {
 void common_hal_bleio_init(void) {
 }
 
-void bleio_user_reset() {
+void bleio_user_reset(void) {
     if (common_hal_bleio_adapter_get_enabled(&common_hal_bleio_adapter_obj)) {
         // Stop any user scanning or advertising.
         common_hal_bleio_adapter_stop_scan(&common_hal_bleio_adapter_obj);
@@ -94,7 +94,7 @@ void bleio_user_reset() {
 }
 
 // Turn off BLE on a reset or reload.
-void bleio_reset() {
+void bleio_reset(void) {
     // Set this explicitly to save data.
     common_hal_bleio_adapter_obj.base.type = &bleio_adapter_type;
     if (!common_hal_bleio_adapter_get_enabled(&common_hal_bleio_adapter_obj)) {
diff --git a/ports/nordic/common-hal/audiopwmio/PWMAudioOut.c b/ports/nordic/common-hal/audiopwmio/PWMAudioOut.c
index 53be11bcd7485..b48a03e09c601 100644
--- a/ports/nordic/common-hal/audiopwmio/PWMAudioOut.c
+++ b/ports/nordic/common-hal/audiopwmio/PWMAudioOut.c
@@ -136,7 +136,7 @@ static void audiopwmout_background_obj(audiopwmio_pwmaudioout_obj_t *self) {
     }
 }
 
-void audiopwmout_background() {
+void audiopwmout_background(void) {
     // Check the NVIC first because it is part of the CPU and fast to read.
     if (!NVIC_GetPendingIRQ(PWM0_IRQn) &&
         !NVIC_GetPendingIRQ(PWM1_IRQn) &&
diff --git a/ports/nordic/common-hal/microcontroller/__init__.c b/ports/nordic/common-hal/microcontroller/__init__.c
index 38a3b1ee60938..fbb40afe0449d 100644
--- a/ports/nordic/common-hal/microcontroller/__init__.c
+++ b/ports/nordic/common-hal/microcontroller/__init__.c
@@ -35,7 +35,7 @@ void common_hal_mcu_delay_us(uint32_t delay) {
 
 static volatile uint32_t nesting_count = 0;
 static uint8_t is_nested_critical_region;
-void common_hal_mcu_disable_interrupts() {
+void common_hal_mcu_disable_interrupts(void) {
     if (nesting_count == 0) {
         // Unlike __disable_irq(), this should only be called the first time
         // "is_nested_critical_region" is sd's equivalent of our nesting count
@@ -51,7 +51,7 @@ void common_hal_mcu_disable_interrupts() {
     nesting_count++;
 }
 
-void common_hal_mcu_enable_interrupts() {
+void common_hal_mcu_enable_interrupts(void) {
     if (nesting_count == 0) {
         // This is very very bad because it means there was mismatched disable/enables.
         reset_into_safe_mode(SAFE_MODE_INTERRUPT_ERROR);
diff --git a/ports/renode/Makefile b/ports/renode/Makefile
index 22e32f9d2fb6e..92541c03e77b9 100644
--- a/ports/renode/Makefile
+++ b/ports/renode/Makefile
@@ -21,7 +21,7 @@ INC += \
 CFLAGS += -ggdb3 -Os
 
 DISABLE_WARNINGS = -Wno-cast-align
-CFLAGS += $(INC) -Wall -Werror -std=gnu11 -fshort-enums $(BASE_CFLAGS) $(CFLAGS_MOD) $(COPT) $(DISABLE_WARNINGS) -Werror=missing-prototypes
+CFLAGS += $(INC) -Wall -Werror -std=gnu11 -fshort-enums $(BASE_CFLAGS) $(CFLAGS_MOD) $(COPT) $(DISABLE_WARNINGS) -Werror=missing-prototypes -Werror=old-style-definition
 
 CFLAGS += \
 	-march=armv6-m \
diff --git a/ports/renode/supervisor/port.c b/ports/renode/supervisor/port.c
index dae32b7bb8f2d..d12ebd2efddb9 100644
--- a/ports/renode/supervisor/port.c
+++ b/ports/renode/supervisor/port.c
@@ -210,7 +210,7 @@ __attribute__((used)) void HardFault_Handler(void) {
     }
 }
 
-void port_yield() {
+void port_yield(void) {
 }
 
 void port_boot_info(void) {
diff --git a/ports/silabs/boards/devkit_xg24_brd2601b/sensor.c b/ports/silabs/boards/devkit_xg24_brd2601b/sensor.c
index aaf698a6cb27f..1c991a4caafc7 100644
--- a/ports/silabs/boards/devkit_xg24_brd2601b/sensor.c
+++ b/ports/silabs/boards/devkit_xg24_brd2601b/sensor.c
@@ -75,7 +75,7 @@ static mp_obj_t sensor_init(mp_obj_t i2c_in) {
     return mp_const_true;
 }
 
-static mp_obj_t sensor_deinit() {
+static mp_obj_t sensor_deinit(void) {
 
     sl_sensor_hall_deinit();
     sl_sensor_lux_deinit();
diff --git a/ports/silabs/common-hal/_bleio/CharacteristicBuffer.c b/ports/silabs/common-hal/_bleio/CharacteristicBuffer.c
index d6a64db3a2e95..d08fa765c4f71 100644
--- a/ports/silabs/common-hal/_bleio/CharacteristicBuffer.c
+++ b/ports/silabs/common-hal/_bleio/CharacteristicBuffer.c
@@ -156,7 +156,7 @@ bool common_hal_bleio_characteristic_buffer_connected(
                    common_hal_bleio_connection_get_connected(self->characteristic->service->connection)));
 }
 
-void reset_characteristic_buffer_list() {
+void reset_characteristic_buffer_list(void) {
     // Remove characteristic_buffer list
     memset(bleio_characteristic_buffer_list.data, 0,
         sizeof(bleio_characteristic_buffer_list.data));
diff --git a/ports/silabs/common-hal/_bleio/PacketBuffer.c b/ports/silabs/common-hal/_bleio/PacketBuffer.c
index 1d2b140f7efef..dec833fe62f85 100644
--- a/ports/silabs/common-hal/_bleio/PacketBuffer.c
+++ b/ports/silabs/common-hal/_bleio/PacketBuffer.c
@@ -391,7 +391,7 @@ void common_hal_bleio_packet_buffer_deinit(bleio_packet_buffer_obj_t *self) {
 }
 
 // Remove packet_buffer list when reload
-void reset_packet_buffer_list() {
+void reset_packet_buffer_list(void) {
     // Remove packet_buffer list
     memset(bleio_packet_buffer_list.data, 0,
         sizeof(bleio_packet_buffer_list.data));
diff --git a/ports/silabs/common-hal/_bleio/Service.c b/ports/silabs/common-hal/_bleio/Service.c
index 694841e66284f..abc8ffe74edff 100644
--- a/ports/silabs/common-hal/_bleio/Service.c
+++ b/ports/silabs/common-hal/_bleio/Service.c
@@ -230,7 +230,7 @@ void common_hal_bleio_service_add_characteristic(bleio_service_obj_t *self,
 }
 
 // Remove dynamic service when reload
-void reset_dynamic_service() {
+void reset_dynamic_service(void) {
 
     uint16_t gattdb_session;
     uint8_t svc_index;
diff --git a/ports/silabs/common-hal/_bleio/__init__.c b/ports/silabs/common-hal/_bleio/__init__.c
index a9ad7b32c36ba..c6ae8a0b606c8 100644
--- a/ports/silabs/common-hal/_bleio/__init__.c
+++ b/ports/silabs/common-hal/_bleio/__init__.c
@@ -54,7 +54,7 @@ const osMutexAttr_t bluetooth_connection_mutex_attr = {
 void common_hal_bleio_init(void) {
 }
 
-void bleio_user_reset() {
+void bleio_user_reset(void) {
     // Stop any user scanning or advertising.
     common_hal_bleio_adapter_stop_scan(&common_hal_bleio_adapter_obj);
     common_hal_bleio_adapter_stop_advertising(&common_hal_bleio_adapter_obj);
@@ -63,7 +63,7 @@ void bleio_user_reset() {
     supervisor_bluetooth_background();
 }
 
-void bleio_reset() {
+void bleio_reset(void) {
     reset_dynamic_service();
     reset_packet_buffer_list();
     reset_characteristic_buffer_list();
diff --git a/ports/stm/Makefile b/ports/stm/Makefile
index baea6893a33d9..9db283767e0ec 100755
--- a/ports/stm/Makefile
+++ b/ports/stm/Makefile
@@ -51,7 +51,7 @@ CFLAGS += -ftree-vrp
 # STM32 MCU series must be defined. See supervisor/linker.h
 C_DEFS = -D$(MCU_PACKAGE) -DUSE_HAL_DRIVER -DUSE_FULL_LL_DRIVER -D$(MCU_VARIANT) -DSTM32$(MCU_SERIES)
 
-CFLAGS += $(INC) -Werror -Wall -std=gnu11 -fshort-enums $(BASE_CFLAGS) $(C_DEFS) $(CFLAGS_MOD) $(COPT) -nostdlib -nostartfiles
+CFLAGS += $(INC) -Werror -Wall -std=gnu11 -fshort-enums $(BASE_CFLAGS) $(C_DEFS) $(CFLAGS_MOD) $(COPT) -nostdlib -nostartfiles -Werror=old-style-definition
 
 # Undo some warnings.
 # STM32 HAL uses undefined preprocessor variables, shadowed variables, casts that change alignment reqs
diff --git a/ports/stm/common-hal/sdioio/SDCard.c b/ports/stm/common-hal/sdioio/SDCard.c
index a5c8b0404a0fb..92e552c2e8984 100644
--- a/ports/stm/common-hal/sdioio/SDCard.c
+++ b/ports/stm/common-hal/sdioio/SDCard.c
@@ -338,7 +338,7 @@ void common_hal_sdioio_sdcard_never_reset(sdioio_sdcard_obj_t *self) {
     }
 }
 
-void sdioio_reset() {
+void sdioio_reset(void) {
     for (size_t i = 0; i < MP_ARRAY_SIZE(reserved_sdio); i++) {
         if (!never_reset_sdio[i]) {
             reserved_sdio[i] = false;
diff --git a/ports/stm/supervisor/port.c b/ports/stm/supervisor/port.c
index 4aa8c044122ce..c5a1685a7fddc 100644
--- a/ports/stm/supervisor/port.c
+++ b/ports/stm/supervisor/port.c
@@ -285,7 +285,7 @@ void HAL_Delay(uint32_t delay_ms) {
     }
 }
 
-uint32_t HAL_GetTick() {
+uint32_t HAL_GetTick(void) {
     if (SysTick->CTRL != 0) {
         return systick_ms;
     } else {
diff --git a/shared-module/usb_cdc/__init__.c b/shared-module/usb_cdc/__init__.c
index 0248c0f180fdb..9a9f158f28cfb 100644
--- a/shared-module/usb_cdc/__init__.c
+++ b/shared-module/usb_cdc/__init__.c
@@ -356,10 +356,10 @@ size_t usb_vendor_descriptor_length(void) {
 
 static uint8_t *ms_os_20_descriptor = NULL;
 
-size_t vendor_ms_os_20_descriptor_length() {
+size_t vendor_ms_os_20_descriptor_length(void) {
     return ms_os_20_descriptor != NULL ? sizeof(ms_os_20_descriptor_template) : 0;
 }
-uint8_t const *vendor_ms_os_20_descriptor() {
+uint8_t const *vendor_ms_os_20_descriptor(void) {
     return ms_os_20_descriptor;
 }