diff --git a/var/spack/repos/builtin/packages/oce/package.py b/var/spack/repos/builtin/packages/oce/package.py
index 4f9f7b2e12..2d518d83c3 100644
--- a/var/spack/repos/builtin/packages/oce/package.py
+++ b/var/spack/repos/builtin/packages/oce/package.py
@@ -54,6 +54,11 @@ class Oce(Package):
     # https://github.com/tpaviot/oce/commit/61cb965b9ffeca419005bc15e635e67589c421dd.patch
     patch('null.patch', when='@0.16:0.17.1')
 
+    # fix build with Xcode 8 "previous definition of CLOCK_REALTIME"
+    # reported 27 Sep 2016 https://github.com/tpaviot/oce/issues/643
+    if (platform.system() == "Darwin") and (platform.mac_ver()[0] == '10.12'):
+        patch('sierra.patch')
+
     def install(self, spec, prefix):
         options = []
         options.extend(std_cmake_args)
diff --git a/var/spack/repos/builtin/packages/oce/sierra.patch b/var/spack/repos/builtin/packages/oce/sierra.patch
new file mode 100644
index 0000000000..8db3af5e37
--- /dev/null
+++ b/var/spack/repos/builtin/packages/oce/sierra.patch
@@ -0,0 +1,45 @@
+diff --git a/adm/cmake/TKernel/CMakeLists.txt b/adm/cmake/TKernel/CMakeLists.txt
+index 54e241e..72873c6 100644
+--- a/adm/cmake/TKernel/CMakeLists.txt
++++ b/adm/cmake/TKernel/CMakeLists.txt
+@@ -29,7 +29,7 @@ if(WIN32)
+ 	set(TOOLKIT_LIBS ${TOOLKIT_LIBS} ${CSF_SOCKETLibs_LIB} ${CSF_advapi32_LIB} ${CSF_gdi32_LIB} ${CSF_user32_LIB} ${CSF_kernel32_LIB} ${CSF_psapi_LIB})
+ else(WIN32)
+ 	#  An implementation for Mac OS X has been added in src/OSD/gettime_osx.h
+-	if(NOT APPLE)
++	if(APPLE)
+ 		include( CheckFunctionExists )
+ 		check_function_exists( clock_gettime CLOCK_GETTIME_IN_LIBC )
+ 		if(NOT CLOCK_GETTIME_IN_LIBC)
+@@ -40,7 +40,7 @@ else(WIN32)
+ 			endif(NOT CLOCK_GETTIME_IN_LIBRT)
+ 			set(TOOLKIT_LIBS ${TOOLKIT_LIBS} rt)
+ 		endif(NOT CLOCK_GETTIME_IN_LIBC)
+-	endif(NOT APPLE)
++	endif(APPLE)
+ endif(WIN32)
+
+ # Adde-DHAVE_TBB in TKernel in order to benefit from Standard_MMgrTBBalloc
+@@ -70,4 +70,3 @@ if (OCE_BUILD_TKERNEL_WITH_INSTALL_RPATH)
+ 	set_target_properties_install_rpath(${TOOLKIT} "${OCE_INSTALL_LIB_DIR_RPATH};${OCE_INSTALL_PACKAGE_LIB_DIR_RPATH}")
+ 	set_target_properties(${TOOLKIT} PROPERTIES BUILD_WITH_INSTALL_RPATH ON)
+ endif ()
+-
+diff --git a/src/OSD/OSD_Chronometer.cxx b/src/OSD/OSD_Chronometer.cxx
+index 444f844..25970d0 100644
+--- a/src/OSD/OSD_Chronometer.cxx
++++ b/src/OSD/OSD_Chronometer.cxx
+@@ -51,7 +51,7 @@
+   #include <mach/mach.h>
+ #endif
+
+-#if defined(__APPLE__) && defined(__MACH__)
++#if defined(__OOOOPPLE__) && defined(__MACH__)
+ #include "gettime_osx.h"
+ #endif
+
+@@ -283,4 +283,3 @@ void OSD_Chronometer::Show (Standard_Real& user,
+   system = Cumul_sys;
+   if (!StopSav) Start();
+ }
+-