ORDER BY SUBSTR(col, 1, 1), TO_NUMBER(SUBSTR(col, 2, LENGTH(col)));

order by substr(col,1,instr(translate(col,'1234567890','##########'),'#')-1),
         to_number(translate(substr(col,instr(translate(col,'1234567890','##########'),'#')),'0 _','0'))

SQL*Plus: Release 10.2.0.1.0 - Production on Fri Jul 20 11:17:17 2007

create table t ( c varchar2(10) );

insert into t
select * from table
( varchar2_1000_varray_type
  ( 'A1', 'A2', 'A3', 'C11 345', 'b356_123', 'b2_31', 'a_2_DF1', 'B 2_cr3', 'A 1_SR4' )
);

select * from t order by
  regexp_replace( c, '[[:digit:]]', '~' ),
  regexp_replace( c, '[[:alpha:]]', '~' )
;

C
----------
A 1_SR4
A1
A2
A3
B 2_cr3
C11 345
a_2_DF1
b2_31
b356_123

SQL> with t as (select 'A 1_SR4' c from dual union all select 'A 2_RW4' from dual)
  2  select * from t order by
  3    regexp_replace( c, '[[:digit:]]', '~' ),
  4    regexp_replace( c, '[[:alpha:]]', '~' )
  5  ;
C
-------
A 2_RW4
A 1_SR4

2 rows selected.