select
 DBMS_LOB.instr('Hello world','o',-1,1) lob_i,
 instr('Hello world','o',-1,1) std_i
from dual;

SCOTT@orcl_11gR2> select
  2    DBMS_LOB.instr ('Hello world', 'o', -1, 1) lob_i,
  3    instr ('Hello world', 'o', -1, 1) std_i,
  4    dbms_lob.getlength ('Hello world')
  5  	 - dbms_lob.instr (reverse ('Hello world'), 'o', 1, 1)
  6  	 + 1 rev_lob_i
  7  from dual;

     LOB_I      STD_I  REV_LOB_I
---------- ---------- ----------
                    8          8

1 row selected.

SQL> l
  1  declare c clob; i integer;
  2  begin
  3    dbms_lob.CREATETEMPORARY (c, TRUE);
  4    c := rpad (' ', 4000) || rpad (' ', 4000) || rpad (' ', 4000) || 'Hello world';
  5    i := INSTR (c, 'o',-1,1) ;
  6    dbms_output.put_line(i);
  7* end;
SQL> /
12008