SELECT * FROM
(
  SELECT LEVEL lvl FROM DUAL 
  CONNECT BY LEVEL < (SELECT MAX(all_objects.object_id) FROM all_objects)
)
WHERE NOT EXISTS (SELECT NULL FROM all_objects WHERE all_objects.object_id = lvl)

  SELECT LEVEL FROM DUAL  CONNECT BY LEVEL < (SELECT MAX(all_objects.object_id) FROM all_objects)
  MINUS
  SELECT all_objects.object_id FROM all_objects 

SELECT * FROM
(
  SELECT LEVEL lvl FROM DUAL 
  CONNECT BY LEVEL < (SELECT MAX(obj$.obj#) FROM obj$)
)
WHERE NOT EXISTS (SELECT NULL FROM obj$ WHERE obj$.obj# = lvl)

with t(n) as (select object_id from all_objects order by 1)
select to_char(max(n)+1)||'-'||(lead(min(n)) over (order by n-rownum)-1),n-rownum
from t
group by n-rownum
order by n-rownum;

SQL> CREATE TABLE zz AS SELECT object_id FROM all_objects;

SQL> CREATE INDEX zzi ON zz(object_id);

SQL> DECLARE
     l_res NUMBER := 0;
   BEGIN
   
     FOR i IN 1..10000 LOOP
       FOR c IN (
         SELECT object_id + 1 ID FROM (
           SELECT --+ index(zz)
             object_id, LEAD(object_id) OVER (ORDER BY object_id) - 1 next_id
           FROM zz
         ) WHERE next_id - object_id  > 1 AND ROWNUM = 1
       ) LOOP
         l_res := l_res + c.id;
       END LOOP;
     END LOOP;
   END;
   /
PL/SQL procedure successfully completed
Executed in 25.898 seconds
SQL> DECLARE
     l_res NUMBER := 0;
   BEGIN
   
     FOR i IN 1..10000 LOOP
       FOR c IN (
         SELECT ID FROM
         (
           SELECT LEVEL ID FROM DUAL
           CONNECT BY 1=1
         )
         WHERE NOT EXISTS (SELECT NULL FROM zz WHERE zz.object_id = ID) AND ROWNUM = 1
       ) LOOP
         l_res := l_res + c.id;
       END LOOP;
     END LOOP;
   END;
   /
PL/SQL procedure successfully completed
Executed in 3.411 seconds