declare
   tmp int;
begin
   for i in (select * from zz order by object_id)
   loop
      if tmp + 1 <> i.object_id then exit; end if;
      tmp   := i.object_id;
   end loop;
   dbms_output.put_line(tmp + 1);
end;

SQL> declare
  2     tmp   int;
  3  begin
  4
  5     for j in (select *
  6                 from zz
  7                where object_id is not null
  8               order by object_id)
  9     loop
 10
 11        if tmp + 1 <> j.object_id
 12        then
 13           dbms_output.put_line(tmp + 1);
 14        end if;
 15
 16        tmp   := j.object_id;
 17
 18     end loop;
 19
 20  end;
 21  /
1234567

PL/SQL procedure successfully completed.

Elapsed: 00:00:04.86
SQL>
SQL> begin
  2
  3     for c in (select id
  4                 from (select level id
  5                         from dual
  6                       connect by 1 = 1)
  7                where not exists
  8                         (select null
  9                            from zz
 10                           where zz.object_id = id)
 11                  and rownum = 1)
 12     loop
 13        dbms_output.put_line(c.id);
 14     end loop;
 15
 16  end;
 17  /
1234567

PL/SQL procedure successfully completed.

Elapsed: 00:00:07.24

SQL> 
SQL> DECLARE
  2    l_res NUMBER := 0;
  3  BEGIN
  4  
  5    FOR i IN 1..10000 LOOP
  6      FOR c IN (
  7        SELECT object_id + 1 ID FROM (
  8          SELECT --+ index(zz)
  9            object_id, LEAD(object_id) OVER (ORDER BY object_id) - 1 next_id
 10          FROM zz
 11        ) WHERE next_id - object_id  > 1 AND ROWNUM = 1
 12      ) LOOP
 13        l_res := l_res + c.id;
 14      END LOOP;
 15    END LOOP;
 16    dbms_output.put_line(l_res);
 17  END;
 18  /
1010000
PL/SQL procedure successfully completed
Executed in 25.249 seconds
SQL> DECLARE
  2    l_res NUMBER := 0;
  3  BEGIN
  4  
  5    FOR i IN 1..10000 LOOP
  6      FOR c IN (
  7        SELECT ID FROM
  8        (
  9          SELECT LEVEL ID FROM DUAL
 10          CONNECT BY 1=1
 11        )
 12        WHERE NOT EXISTS (SELECT NULL FROM zz WHERE zz.object_id = ID) AND ROWNUM = 1
 13      ) LOOP
 14        l_res := l_res + c.id;
 15      END LOOP;
 16    END LOOP;
 17    dbms_output.put_line(l_res);
 18  END;
 19  /
1010000
PL/SQL procedure successfully completed
Executed in 3.476 seconds
SQL> DECLARE
  2    l_res NUMBER := 0;
  3  BEGIN
  4  
  5    FOR i IN 1..10000 LOOP
  6      FOR c IN (
  7        SELECT rn id FROM (
  8          SELECT --+ index(zz)
  9           object_id, ROWNUM rn FROM zz
 10          ORDER BY object_id
 11        )
 12        WHERE object_id <> rn AND ROWNUM = 1
 13      ) LOOP
 14        l_res := l_res + c.id;
 15      END LOOP;
 16    END LOOP;
 17    dbms_output.put_line(l_res);
 18  END;
 19  /
1010000
PL/SQL procedure successfully completed
Executed in 0.775 seconds

SELECT --+ index(zz)
  9           object_id, ROWNUM rn FROM zz
 10          ORDER BY object_id