diff --git a/Source/cmLoadCommandCommand.cxx b/Source/cmLoadCommandCommand.cxx
index 5a7b30f193..2981ef877e 100644
--- a/Source/cmLoadCommandCommand.cxx
+++ b/Source/cmLoadCommandCommand.cxx
@@ -1,12 +1,12 @@
 /* Distributed under the OSI-approved BSD 3-Clause License.  See accompanying
    file Copyright.txt or https://cmake.org/licensing for details.  */
 
-#if !defined(_WIN32) && !defined(__sun)
+#if !defined(_WIN32) && !defined(__sun) && !defined(__OpenBSD__)
 // POSIX APIs are needed
 // NOLINTNEXTLINE(bugprone-reserved-identifier)
 #  define _POSIX_C_SOURCE 200809L
 #endif
-#if defined(__OpenBSD__) || defined(__FreeBSD__) || defined(__NetBSD__)
+#if defined(__FreeBSD__) || defined(__NetBSD__)
 // For isascii
 // NOLINTNEXTLINE(bugprone-reserved-identifier)
 #  define _XOPEN_SOURCE 700
diff --git a/Source/cmStandardLexer.h b/Source/cmStandardLexer.h
index 1da8c98440..b871f5fca5 100644
--- a/Source/cmStandardLexer.h
+++ b/Source/cmStandardLexer.h
@@ -7,7 +7,8 @@
 // NOLINTNEXTLINE(bugprone-reserved-identifier)
 #  define _XOPEN_SOURCE 600
 #endif
-#if !defined(_POSIX_C_SOURCE) && !defined(_WIN32) && !defined(__sun)
+#if !defined(_POSIX_C_SOURCE) && !defined(_WIN32) && !defined(__sun) &&       \
+  !defined(__OpenBSD__)
 /* POSIX APIs are needed */
 // NOLINTNEXTLINE(bugprone-reserved-identifier)
 #  define _POSIX_C_SOURCE 200809L
@@ -17,7 +18,7 @@
 // NOLINTNEXTLINE(bugprone-reserved-identifier)
 #  define _XOPEN_SOURCE 600
 #endif
-#if defined(__OpenBSD__) || defined(__FreeBSD__) || defined(__NetBSD__)
+#if defined(__FreeBSD__) || defined(__NetBSD__)
 /* For isascii */
 // NOLINTNEXTLINE(bugprone-reserved-identifier)
 #  define _XOPEN_SOURCE 700
diff --git a/Source/cmSystemTools.cxx b/Source/cmSystemTools.cxx
index 0807590770..c1ce7ec989 100644
--- a/Source/cmSystemTools.cxx
+++ b/Source/cmSystemTools.cxx
@@ -1,13 +1,12 @@
 /* Distributed under the OSI-approved BSD 3-Clause License.  See accompanying
    file Copyright.txt or https://cmake.org/licensing for details.  */
 
-#if !defined(_WIN32) && !defined(__sun)
+#if !defined(_WIN32) && !defined(__sun) && !defined(__OpenBSD__)
 // POSIX APIs are needed
 // NOLINTNEXTLINE(bugprone-reserved-identifier)
 #  define _POSIX_C_SOURCE 200809L
 #endif
-#if defined(__OpenBSD__) || defined(__FreeBSD__) || defined(__NetBSD__) ||    \
-  defined(__QNX__)
+#if defined(__FreeBSD__) || defined(__NetBSD__) || defined(__QNX__)
 // For isascii
 // NOLINTNEXTLINE(bugprone-reserved-identifier)
 #  define _XOPEN_SOURCE 700
diff --git a/Source/cmTimestamp.cxx b/Source/cmTimestamp.cxx
index b016530099..056696d4b1 100644
--- a/Source/cmTimestamp.cxx
+++ b/Source/cmTimestamp.cxx
@@ -1,13 +1,12 @@
 /* Distributed under the OSI-approved BSD 3-Clause License.  See accompanying
    file Copyright.txt or https://cmake.org/licensing for details.  */
 
-#if !defined(_WIN32) && !defined(__sun)
+#if !defined(_WIN32) && !defined(__sun) && !defined(__OpenBSD__)
 // POSIX APIs are needed
 // NOLINTNEXTLINE(bugprone-reserved-identifier)
 #  define _POSIX_C_SOURCE 200809L
 #endif
-#if defined(__OpenBSD__) || defined(__FreeBSD__) || defined(__NetBSD__) ||    \
-  defined(__QNX__)
+#if defined(__FreeBSD__) || defined(__NetBSD__) || defined(__QNX__)
 // For isascii
 // NOLINTNEXTLINE(bugprone-reserved-identifier)
 #  define _XOPEN_SOURCE 700