sql> select BLOCK_ID, BYTES, BLOCKS from dba_free_space where TABLESPACE_NAME='tbsp' order by 1;      

  BLOCK_ID      BYTES     BLOCKS
---------- ---------- ----------
      3337 2124414976     259328
    262793  529530880      64640
    327561  319815680      39040
    366697      65536          8
    366713 1156710400     141200
    507913  134152192      16376

6 rows selected.

sql> select segment_name, segment_type, block_id, bytes, blocks from dba_extents where tablespace_name='tbsp' order by 3;

SEGMENT_NAME              SEGMENT_TYPE         BLOCK_ID      BYTES     BLOCKS
------------------------- ------------------ ---------- ---------- ----------
[...]
my_index1                 INDEX                  366705      65536          8

sql> alter index my_index1 rebuild;

Index altered.

sql> select segment_name, segment_type, block_id, bytes, blocks from dba_extents where tablespace_name='tbsp' order by 3;

SEGMENT_NAME              SEGMENT_TYPE         BLOCK_ID      BYTES     BLOCKS
------------------------- ------------------ ---------- ---------- ----------
[...]
my_index1                 INDEX                  366697      65536          8