SQL> create table par_demo
  2    ( d date, z int )
  3  partition by range ( d )
  4  (
  5    partition p_201512 values less than ( date '2016-01-01' ),
  6    partition p_201601 values less than ( date '2016-02-01' ),
  7    partition p_201602 values less than ( date '2016-03-01' )
  8  );

Table created.

SQL> set serverout on
SQL> declare
  2    l_long      long;
  3    l_hi_date   date;
  4    l_next_date date;
  5    l_ddl       long;
  6  begin
  7    select high_value
  8    into   l_long
  9    from   user_tab_partitions
 10    where  table_name = 'PAR_DEMO'
 11    and    partition_position =
 12      ( select max(partition_position)
 13        from   user_tab_partitions
 14        where  table_name = 'PAR_DEMO' );
 15
 16    execute immediate 'select '||l_long||' from dual' into l_hi_date;
 17
 18    l_next_date := add_months(l_hi_date,1);
 19
 20    dbms_output.put_line('New upper bound is '||l_next_date);
 21
 22    l_ddl :=
 23      'alter table par_demo add partition p_'||to_char(l_hi_date,'YYYYMM')||' values less than ( date '''||to_char(l_next_date,'YYYY-MM-DD')||''')';
 24
 25    dbms_output.put_line(l_ddl);
 26    execute immediate l_ddl;
 27  end;
 28  /
New upper bound is 01-APR-16
alter table par_demo add partition p_201603 values less than ( date '2016-04-01')

PL/SQL procedure successfully completed.

SQL> /
New upper bound is 01-MAY-16
alter table par_demo add partition p_201604 values less than ( date '2016-05-01')

PL/SQL procedure successfully completed.

SQL> /
New upper bound is 01-JUN-16
alter table par_demo add partition p_201605 values less than ( date '2016-06-01')

PL/SQL procedure successfully completed.