CREATE TABLE leading_0_test
(
  col_1  VARCHAR2(10)
)

insert into leading_0_test values('00123')

select trim(leading'0' from COL_1),trim(leading'0' from substr(COL_1,1,1)),COL_1 from leading_0_test

SQL> ed
Wrote file afiedt.buf

  1  with t as
  2  (select '00123' a from dual union all select '12345' from dual)
  3  select case when substr(a,1,1)='0' then substr(a,2,length(a))
  4*                  else a end output from t
SQL> /

OUTPU                                                                           
-----                                                                           
0123                                                                            
12345   

column new_col1 format a10
With yourtable As
( Select cast('00123' as varchar2(10)) col1 from dual union all
  Select cast('9876'  as varchar2(10)) col1 from dual union all
  Select cast('0789'  as varchar2(10)) col1 from dual union all
  Select cast('AB00A' as varchar2(10)) col1 from dual union all
  Select cast('00012' as varchar2(10)) col1 from dual
)
select col1
     , regexp_replace(col1, '^0','') new_col1
from   yourtable
/