diff --git a/vlib/builtin/int_test.v b/vlib/builtin/int_test.v
index de22637478..455ac37885 100644
--- a/vlib/builtin/int_test.v
+++ b/vlib/builtin/int_test.v
@@ -117,6 +117,20 @@ fn test_hex() {
 fn test_oct() {
 	x1 := 0o12
 	assert x1 == 10
-	x2 := 012
-	assert x2 == 10
+	x2 := 00000o350
+	assert x2 == 232
+	x3 := 000o00073
+	assert x3 == 59
+	x4 := 00000000
+	assert x4 == 0
+	x5 := 00000195
+	assert x5 == 195
+	x6 := -0o744
+	assert x6 == -484
+	x7 := -000o000042
+	assert x7 == -34
+	x8 := -0000112
+	assert x8 == -112
+	x9 := -000
+	assert x9 == 0
 }
diff --git a/vlib/compiler/scanner.v b/vlib/compiler/scanner.v
index b7ac8761d6..1fd24060fb 100644
--- a/vlib/compiler/scanner.v
+++ b/vlib/compiler/scanner.v
@@ -346,6 +346,19 @@ fn (s mut Scanner) scan() ScanRes {
 	}
 	// `123`, `.123`
 	else if c.is_digit() || (c == `.` && nextc.is_digit()) {
+		if !s.inside_string {
+		    // In C ints with `0` prefix are octal (in V they're decimal), so discarding heading zeros is needed.
+			mut start_pos := s.pos
+			for start_pos < s.text.len && s.text[start_pos] == `0` {
+				start_pos++
+			}
+			mut prefix_zero_num := start_pos - s.pos  // how many prefix zeros should be jumped
+			// for 0b, 0o, 0x the heading zero shouldn't be jumped
+			if c == `0` && start_pos < s.text.len && !s.text[start_pos].is_digit() {
+				prefix_zero_num--
+			}
+			s.pos += prefix_zero_num  // jump these zeros
+		}
 		num := s.ident_number()
 		return scan_res(.number, num)
 	}
diff --git a/vlib/os/os.v b/vlib/os/os.v
index 1d760085c9..f10a40acb0 100644
--- a/vlib/os/os.v
+++ b/vlib/os/os.v
@@ -290,13 +290,13 @@ pub fn open_file(path string, mode string, options ...int) ?File {
 		}
 	}
 
-	mut permission := 0666
+	mut permission := 0o666
 	if options.len > 0 {
 		permission = options[0]
 	}
 
 	$if windows {
-		if permission < 0600 {
+		if permission < 0o600 {
 			permission = 0x0100
 		}
 		else {
diff --git a/vlib/os/os_test.v b/vlib/os/os_test.v
index b3cec0fa9d..ca59406e02 100644
--- a/vlib/os/os_test.v
+++ b/vlib/os/os_test.v
@@ -33,11 +33,11 @@ fn test_unsetenv() {
 fn test_open_file() {
   filename := './test1.txt'
   hello := 'hello world!'
-  os.open_file(filename, "r+", 0666) or {
+  os.open_file(filename, "r+", 0o666) or {
     assert err == "No such file or directory"
   }
 
-  mut file := os.open_file(filename, "w+", 0666) or { panic(err) }
+  mut file := os.open_file(filename, "w+", 0o666) or { panic(err) }
   file.write(hello)
   file.close()
 
@@ -278,11 +278,11 @@ fn test_is_executable_writable_readable() {
   f.close()
 
   $if !windows {
-    os.chmod(file_name, 0600) // mark as readable && writable, but NOT executable
+    os.chmod(file_name, 0o600) // mark as readable && writable, but NOT executable
     assert os.is_writable(file_name)
     assert os.is_readable(file_name)
     assert !os.is_executable(file_name)
-    os.chmod(file_name, 0700) // mark as executable too
+    os.chmod(file_name, 0o700) // mark as executable too
     assert os.is_executable(file_name)
   } $else {
     assert os.is_writable(file_name)
diff --git a/vlib/strconv/atoi_test.v b/vlib/strconv/atoi_test.v
index 6c34fd6462..dbdd0a48bf 100644
--- a/vlib/strconv/atoi_test.v
+++ b/vlib/strconv/atoi_test.v
@@ -11,7 +11,7 @@ fn test_atoi() {
 fn test_parse_int() {
 	// Different bases
 	assert strconv.parse_int('16', 16, 0) == 0x16
-	assert strconv.parse_int('16', 8, 0) == 016
+	assert strconv.parse_int('16', 8, 0) == 0o16
 	assert strconv.parse_int('11', 2, 0) == 3
 	// Different bit sizes
 	assert strconv.parse_int('127', 10, 8) == 127
diff --git a/vlib/v/gen/x64/elf.v b/vlib/v/gen/x64/elf.v
index df49205584..fdcdc83609 100644
--- a/vlib/v/gen/x64/elf.v
+++ b/vlib/v/gen/x64/elf.v
@@ -92,7 +92,7 @@ pub fn (g mut Gen) generate_elf_footer() {
 	mut f := os.create(g.out_name) or {
 		panic(err)
 	}
-	os.chmod(g.out_name, 0775) // make it an executable
+	os.chmod(g.out_name, 0o775) // make it an executable
 	f.write_bytes(g.buf.data, g.buf.len)
 	f.close()
 	println('x64 elf binary has been successfully generated')
diff --git a/vlib/v/scanner/scanner.v b/vlib/v/scanner/scanner.v
index fff6d8926c..03ca391350 100644
--- a/vlib/v/scanner/scanner.v
+++ b/vlib/v/scanner/scanner.v
@@ -340,6 +340,19 @@ pub fn (s mut Scanner) scan() token.Token {
 	}
 	// `123`, `.123`
 	else if c.is_digit() || (c == `.` && nextc.is_digit()) {
+		if !s.inside_string {
+		    // In C ints with `0` prefix are octal (in V they're decimal), so discarding heading zeros is needed.
+			mut start_pos := s.pos
+			for start_pos < s.text.len && s.text[start_pos] == `0` {
+				start_pos++
+			}
+			mut prefix_zero_num := start_pos - s.pos  // how many prefix zeros should be jumped
+			// for 0b, 0o, 0x the heading zero shouldn't be jumped
+			if c == `0` && start_pos < s.text.len && !s.text[start_pos].is_digit() {
+				prefix_zero_num--
+			}
+			s.pos += prefix_zero_num  // jump these zeros
+		}
 		num := s.ident_number()
 		return s.scan_res(.number, num)
 	}