Use the proper OS2 test symbol: PYOS_OS2.
diff --git a/Include/osdefs.h b/Include/osdefs.h
index 594fc10..1a00fc0 100644
--- a/Include/osdefs.h
+++ b/Include/osdefs.h
@@ -46,7 +46,7 @@
/* Mod by chrish: QNX has WATCOM, but isn't DOS */
#if !defined(__QNX__)
-#if defined(MS_WINDOWS) || defined(__BORLANDC__) || defined(__WATCOMC__) || defined(__DJGPP__) || defined(__TOS_OS2__)
+#if defined(MS_WINDOWS) || defined(__BORLANDC__) || defined(__WATCOMC__) || defined(__DJGPP__) || defined(PYOS_OS2)
#define SEP '\\'
#define ALTSEP '/'
#define MAXPATHLEN 256