create table t ( order_no int );

insert into t values (20001200);
insert into t values (20001201);
insert into t values (20001202);
insert into t values (20001203);
insert into t values (20001204);
insert into t values (20001205);
insert into t values (20001206);
insert into t values (20001209);
insert into t values (20001210);
insert into t values (20001212);
insert into t values (20001214);
insert into t values (20001218);

with rws as (
  select 20001200 + rownum - 1 r from dual
  connect by level <= (select max(order_no)-min(order_no) from t)
)
  select r from rws
  where  not exists (select * from t where rws.r = t.order_no)
  order  by r;

         R
----------
  20001207
  20001208
  20001211
  20001213
  20001215
  20001216
  20001217