diff --git a/documentation/release_6.3.htm b/documentation/release_6.3.htm
index ee330680c..84a6c074a 100644
--- a/documentation/release_6.3.htm
+++ b/documentation/release_6.3.htm
@@ -83,6 +83,7 @@ <h3>Build system</h3>
   <li>Enable more diagnostics in CMake when finding CERN's ROOT (we used to silence them)<br>
     <a href=https://github.com/UCL/STIR/pull/1552>PR #1552</a>
   </li>
+  <li>Use OpenMP by default</li>
 </ul>
 
 <h3>Known problems</h3>
diff --git a/src/CMakeLists.txt b/src/CMakeLists.txt
index 6eca7a815..7d914bdd6 100644
--- a/src/CMakeLists.txt
+++ b/src/CMakeLists.txt
@@ -14,7 +14,7 @@
 option(STIR_MPI 
        "Compile with MPI" OFF)
 option(STIR_OPENMP 
-       "Compile with OpenMP" OFF)
+       "Compile with OpenMP" ON)
 
 option(BUILD_TESTING 
        "Build test programs" ON)
@@ -188,13 +188,6 @@ check_cxx_source_compiles("
        HAVE_STD_SHARED_PTR
         )
 
-if(HAVE_STD_SHARED_PTR)
-    option(STIR_USE_BOOST_SHARED_PTR "Use boost::shared_ptr as opposed to std::shared_ptr (not recommended)" OFF)
-    mark_as_advanced(STIR_USE_BOOST_SHARED_PTR)
-else()
-    set(STIR_USE_BOOST_SHARED_PTR ON)
-endif()
-
 # Byte order
 if (CMAKE_VERSION VERSION_GREATER_EQUAL 3.20)
     # Use CMake to find byte order of target system.
diff --git a/src/include/stir/WienerImageFilter2D.h b/src/include/stir/WienerImageFilter2D.h
index 36463362a..89d369dae 100644
--- a/src/include/stir/WienerImageFilter2D.h
+++ b/src/include/stir/WienerImageFilter2D.h
@@ -20,12 +20,12 @@
 */
 
 #ifndef __stir_WienerImageFilter2D_H__
-#  define __stir_WienerImageFilter2_H__
+#define __stir_WienerImageFilter2D_H__
 
-#  include "stir/DataProcessor.h"
-#  include "stir/WienerArrayFilter2D.h"
-#  include "stir/DiscretisedDensity.h"
-#  include "stir/RegisteredParsingObject.h"
+#include "stir/DataProcessor.h"
+#include "stir/WienerArrayFilter2D.h"
+#include "stir/DiscretisedDensity.h"
+#include "stir/RegisteredParsingObject.h"
 
 START_NAMESPACE_STIR
 
diff --git a/src/include/stir/num_threads.h b/src/include/stir/num_threads.h
index 7e65104f7..5e30a68e7 100644
--- a/src/include/stir/num_threads.h
+++ b/src/include/stir/num_threads.h
@@ -16,6 +16,9 @@
   \author Kris Thielemans
 */
 
+#ifndef __stir_num_threads_h__
+#define __stir_num_threads_h__
+
 #include "stir/common.h"
 
 START_NAMESPACE_STIR
@@ -65,3 +68,5 @@ int get_default_num_threads();
 void set_default_num_threads();
 
 END_NAMESPACE_STIR
+
+#endif
diff --git a/src/include/stir/shared_ptr.h b/src/include/stir/shared_ptr.h
index a24edfe3c..c4ee12bd3 100644
--- a/src/include/stir/shared_ptr.h
+++ b/src/include/stir/shared_ptr.h
@@ -5,9 +5,7 @@
   \ingroup buildblock
 
   \brief Import of std::shared_ptr, std::dynamic_pointer_cast and
-  std::static_pointer_cast (or corresponding boost versions if
-  STIR_USE_BOOST_SHARED_PTR is set, i.e. normally when std::shared_ptr doesn't exist)
-  into the stir namespace.
+  std::static_pointer_cast into the stir namespace.
 */
 /*
     Copyright (C) 2011-07-01 - 2012, Kris Thielemans
@@ -22,30 +20,15 @@
 #define __stir_SHARED_PTR__
 
 #include "stir/common.h"
-#if defined(STIR_USE_BOOST_SHARED_PTR)
-#  include "boost/shared_ptr.hpp"
-#  include "boost/make_shared.hpp"
-#  include "boost/pointer_cast.hpp"
-namespace stir
-{
-using boost::shared_ptr;
-using boost::dynamic_pointer_cast;
-using boost::static_pointer_cast;
-//! work-around for using std::make_shared on old compilers
-#  define MAKE_SHARED boost::make_shared
-} // namespace stir
-#else
-
-#  include <memory>
+#include <memory>
 
 namespace stir
 {
 using std::shared_ptr;
 using std::dynamic_pointer_cast;
 using std::static_pointer_cast;
-//! work-around for using std::make_shared on old compilers
-#  define MAKE_SHARED std::make_shared
+//! work-around for using std::make_shared on old compilers (deprecated)
+#define MAKE_SHARED std::make_shared
 } // namespace stir
-#endif
 
 #endif