How much disk space can I save?
Simply execute the following statement to have a look at how much space your tablespaces currently occupy and how much space you could save. The statement provides you with the current size of each tablespace as well as the size you could shrink it to and the space you would save when shrinking it to that size:
select file_name, ceil( (nvl(hwm,1)*8192)/1024/1024 ) SHRINK_TO, ceil( blocks*8192/1024/1024) CURRENT_SIZE, ceil( blocks*8192/1024/1024) - ceil( (nvl(hwm,1)*8192)/1024/1024 ) SAVINGS from dba_data_files a, ( select file_id, max(block_id+blocks-1) hwm from dba_extents group by file_id ) b where a.file_id = b.file_id(+)
Save the space!
Then run the following statement to shrink a tablespace to a specified size.
alter database datafile '/path/to/data/file.dbf' resize 100m;
I found this very helpful hint at stackexchange: http://dba.stackexchange.com/questions/23822/how-do-we-free-up-disk-space-from-allocated-namespace-ora-03297