diff --git a/packages/PLStream/pl-global.h b/packages/PLStream/pl-global.h
index d6b4ded2b..e1fa1b173 100644
--- a/packages/PLStream/pl-global.h
+++ b/packages/PLStream/pl-global.h
@@ -76,6 +76,11 @@ typedef struct {
     atom_t     *for_code[256];		/* code --> one-char-atom */
   } atoms;
 
+  struct
+  {
+    int		optimise;		/* -O: optimised compilation */
+  } cmdline;
+
   struct
   { ExtensionCell _ext_head;		/* head of registered extensions */
     ExtensionCell _ext_tail;		/* tail of this chain */
@@ -167,6 +172,7 @@ typedef struct PL_local_data {
     occurs_check_t occurs_check;	/* Unify and occurs check */
   } feature;
 
+
   source_location read_source;		/* file, line, char of last term */
 
   struct
diff --git a/packages/PLStream/pl-prologflag.c b/packages/PLStream/pl-prologflag.c
index 3c135e767..6460f5000 100644
--- a/packages/PLStream/pl-prologflag.c
+++ b/packages/PLStream/pl-prologflag.c
@@ -994,7 +994,6 @@ initPrologFlags(void)
 #if defined(HAVE_GETPID) || defined(EMULATE_GETPID)
   setPrologFlag("pid", FT_INTEGER|FF_READONLY, getpid());
 #endif
-  setPrologFlag("optimise", FT_BOOL, GD->cmdline.optimise, PLFLAG_OPTIMISE);
   setPrologFlag("generate_debug_info", FT_BOOL,
 	     truePrologFlag(PLFLAG_DEBUGINFO), PLFLAG_DEBUGINFO);
   setPrologFlag("last_call_optimisation", FT_BOOL, TRUE, PLFLAG_LASTCALL);
@@ -1100,6 +1099,8 @@ initPrologFlags(void)
   }
 #endif
 #endif /* YAP_PROLOG */
+  /* Flags copied by YAP */
+  setPrologFlag("optimise", FT_BOOL, GD->cmdline.optimise, PLFLAG_OPTIMISE);
   /* FLAGS used by PLStream */
   setPrologFlag("tty_control", FT_BOOL|FF_READONLY,
 	     truePrologFlag(PLFLAG_TTY_CONTROL), PLFLAG_TTY_CONTROL);
diff --git a/pl/flags.yap b/pl/flags.yap
index a1b039f89..0982737e8 100644
--- a/pl/flags.yap
+++ b/pl/flags.yap
@@ -67,6 +67,13 @@ yap_flag(fileerrors,X) :-
 yap_flag(fileerrors,X) :-
 	'$swi_set_prolog_flag'(fileerrors, X).
 
+% -O optimisation
+yap_flag(optimise,X) :-
+	var(X), !,
+	'$swi_current_prolog_flag'(optimise, X).
+yap_flag(optimise,X) :-
+	'$swi_set_prolog_flag'(optimise, X).
+
 % control garbage collection
 yap_flag(gc,V) :-
 	var(V), !,
@@ -834,6 +841,7 @@ yap_flag(dialect,yap).
 '$yap_system_flag'(n_of_integer_keys_in_db).
 '$yap_system_flag'(open_expands_filename).
 '$yap_system_flag'(open_shared_object).
+'$yap_system_flag'(optimise).
 '$yap_system_flag'(profiling).
 '$yap_system_flag'(prompt_alternatives_on).
 '$yap_system_flag'(readline).