CREATE OR REPLACE FUNCTION t_func(val IN OUT INTEGER) RETURN NUMBER IS
tmpVar NUMBER;
BEGIN
   tmpVar :=  0 ;
   RETURN tmpVar;
END t_func;

select t_func( 1 ) from dual;
ORA- 06572 : Function string has out arguments 

CREATE OR REPLACE FUNCTION dummy_fnc(p_dummy_par IN OUT NUMBER)
  RETURN NUMBER IS
  l_dummy_var NUMBER;
BEGIN
  l_dummy_var := p_dummy_par * - 1 ;

  RETURN l_dummy_var;
END dummy_fnc;

alexey@devrkc>;exec :x := dummy_fnc( 1 );
BEGIN :x := dummy_fnc( 1 ); END;

                      *
ERROR at line  1 :
ORA- 06550 : line  1 , column  23 :
PLS- 00363 : expression '1' cannot be used as an assignment target
ORA- 06550 : line  1 , column  7 :
PL/SQL: Statement ignored

CREATE OR REPLACE FUNCTION scott.dummy_fnc(p_dummy_par IN OUT NUMBER)
  RETURN NUMBER IS
  l_dummy_var NUMBER;
BEGIN
  l_dummy_var := p_dummy_par * - 1 ;
  p_dummy_par := p_dummy_par +  10 ;
  RETURN l_dummy_var;
END dummy_fnc;
/
declare
v_out NUMBER;
v_in_out NUMBER :=  1 ;
begin
v_out := scott.dummy_fnc(v_in_out);
dbms_output.put_line('v_out '||v_out);
dbms_output.put_line('v_in_out '||v_in_out);
end;
/
v_out - 1 
v_in_out  11