diff --git a/src/libCom/osi/os/RTEMS/osdThread.c b/src/libCom/osi/os/RTEMS/osdThread.c
index 207961d45..6d156007f 100644
--- a/src/libCom/osi/os/RTEMS/osdThread.c
+++ b/src/libCom/osi/os/RTEMS/osdThread.c
@@ -478,9 +478,11 @@ void epicsThreadOnceOsd(epicsThreadOnceId *id, void(*func)(void *), void *arg)
     if (!initialized) epicsThreadInit();
     epicsMutexMustLock(onceMutex);
     if (*id == 0) {
-            *id = -1;
-            func(arg);
-            *id = 1;
+        *id = -1;
+        epicsMutexUnlock(onceMutex);
+        func(arg);
+        epicsMutexMustLock(onceMutex);
+        *id = 1;
     } else
         assert(*id > 0 /* func() called epicsThreadOnce() with same id */);
     epicsMutexUnlock(onceMutex);
diff --git a/src/libCom/osi/os/WIN32/osdThread.c b/src/libCom/osi/os/WIN32/osdThread.c
index 04a14f9d2..782db6174 100644
--- a/src/libCom/osi/os/WIN32/osdThread.c
+++ b/src/libCom/osi/os/WIN32/osdThread.c
@@ -1004,7 +1004,9 @@ epicsShareFunc void epicsShareAPI epicsThreadOnceOsd (
 
     if ( *id == 0 ) {
         *id = -1;
+        LeaveCriticalSection ( & pGbl->mutex );
         ( *func ) ( arg );
+        EnterCriticalSection ( & pGbl->mutex );
         *id = 1;
     } else
         assert(*id > 0 /* func() called epicsThreadOnce() with same id */);
diff --git a/src/libCom/osi/os/vxWorks/osdThread.c b/src/libCom/osi/os/vxWorks/osdThread.c
index e6df27f8b..c4cea9563 100644
--- a/src/libCom/osi/os/vxWorks/osdThread.c
+++ b/src/libCom/osi/os/vxWorks/osdThread.c
@@ -116,7 +116,9 @@ void epicsThreadOnceOsd(epicsThreadOnceId *id, void (*func)(void *), void *arg)
     assert(semTake(epicsThreadOnceMutex,WAIT_FOREVER)==OK);
     if (*id == 0) { /*  0 => first call */
         *id = -1;   /* -1 => func() active */
+        semGive(epicsThreadOnceMutex);
         func(arg);
+        assert(semTake(epicsThreadOnceMutex,WAIT_FOREVER)==OK);
         *id = +1;   /* +1 => func() done */
     } else
         assert(*id > 0 /* func() called epicsThreadOnce() with same id */);