diff --git a/firmware/alu/tests/toy_16.vhd b/firmware/alu/tests/toy_16.vhd
index 84f0ccf..8f6cbbf 100644
--- a/firmware/alu/tests/toy_16.vhd
+++ b/firmware/alu/tests/toy_16.vhd
@@ -126,6 +126,13 @@ begin
         wait for period;
         assert result = "0111110110011101" report "ADD failed";
 
+        info("ADD-O");
+        accu <= "1111111111111111";
+        ram <=  "0000000000000001"; carry_in <= '0';
+        func <= "0011";
+        wait for period;
+        assert result = "0000000000000000" report "ADD-O failed";
+
         info("SUB");
         accu <= "0100010000110011";
         ram <=  "0011100000101010"; carry_in <= '0';
@@ -133,6 +140,13 @@ begin
         wait for period;
         assert result = "0000110000001001" report "SUB failed";
 
+        info("SUB-O");
+        accu <= "0000000000000000";
+        ram <=  "0000000000000001"; carry_in <= '0';
+        func <= "0100";
+        wait for period;
+        assert result = "1111111111111111" report "SUB-O failed";
+
         info("SUB-B");
         accu <= "0100010010110011";
         ram <=  "0011100011101010"; carry_in <= '0';