oracle can take care of the indexes by itself in most cases but the problem is with the cost based optimizer which oracle uses to build query execution plans after a delete operation these index structure ( b* tree )would get out of sequence and hence if you do not rebuild the index and dont anakyze your indexes after a massive delete operation then your queries would run slowly.

By the way there is difference between dbase and oracle although they are databses in the same way you cant compare a lizard and a dinasours although both belong to the reptile family right

hope thsi helps