Hex to Dec
=================================================================

select to_number('ff','XX') from dual;

TO_NUMBER('FF','XX')
--------------------
                 255

SQL> select to_number('ff','xx') from dual;

TO_NUMBER('FF','XX')
--------------------
                 255
SQL> select to_number('ff','x') from dual;
select to_number('ff','x') from dual
                 *
ERROR at line 1:
ORA-01722: invalid number


Dec to Hex
=================================================================

SQL>select to_char(255,'XX') from dual;

TO_
---
 FF

SQL> select to_char(255,'xx') from dual

TO_
---
 ff