Better to use a sequence:

create or replace sequence prod_seq minvalue 1 nomaxvalue start with x
+xxx increment by 1 cache 20 noorder nocycle
[download]

Then your insert select can use the nextval function:

insert into prod (ss)
 select prod_seq.nextval from dual
[download]

You need to create the sequence only once, but be sure to set the "start with xxxx" to a value higher than anything in your table.

Paul