diff --git a/src/Makefile b/src/Makefile
index 80e86fe..02b5690 100644
--- a/src/Makefile
+++ b/src/Makefile
@@ -9,8 +9,10 @@ endif
 
 BUILDDATE := `date +"%Y%m%d"`
 
-CFLAGS += -Wall -Wextra -Wno-char-subscripts\
-	-O2 -std=gnu99 -g3 -DBUILDDATE=\"$(BUILDDATE)\"\
+OPT_FLAGS ?= -O2
+
+CFLAGS += -Wall -Wextra -Werror -Wno-char-subscripts\
+	$(OPT_FLAGS) -std=gnu99 -g3 -DBUILDDATE=\"$(BUILDDATE)\"\
 	-I. -Iinclude -Iplatforms/common -I$(PLATFORM_DIR) \
 	-DVERSION_SUFFIX=\"`../scripts/setlocalversion`\" -MD
 
diff --git a/src/platforms/stlink/Makefile.inc b/src/platforms/stlink/Makefile.inc
index 40635cf..689c11e 100644
--- a/src/platforms/stlink/Makefile.inc
+++ b/src/platforms/stlink/Makefile.inc
@@ -2,6 +2,7 @@ CROSS_COMPILE ?= arm-none-eabi-
 CC = $(CROSS_COMPILE)gcc
 OBJCOPY = $(CROSS_COMPILE)objcopy
 
+OPT_FLAGS = -Os
 CFLAGS += -mcpu=cortex-m3 -mthumb \
 	-DSTM32F1 -DDISCOVERY_STLINK -I../libopencm3/include \
 	-I platforms/stm32