diff --git a/apps/ocsp.c b/apps/ocsp.c
index 1ae4cf2ff..6f36edcc5 100644
--- a/apps/ocsp.c
+++ b/apps/ocsp.c
@@ -57,6 +57,9 @@
  */
 #ifndef OPENSSL_NO_OCSP
 
+#define _XOPEN_SOURCE_EXTENDED	/* So fd_set and friends get properly defined
+				   on OpenVMS */
+
 #include <stdio.h>
 #include <stdlib.h>
 #include <string.h>