high water mark??

I want to reduce the size of the tablespace. Before I do that, I want to see
the high water mark of the tablespace. What is the object name that I can query
to find the high water mark?

Tablespaces don't have a HWM. What exactly are you trying to do?

You could see the HWM for all the tables in tha Tablespace.
Sum this for all the tables.

you can see a sort of HWM for datafiles



rem
rem Remarks: minimal size of a datafile is 2 Oracle blocks
rem resizing should always be a multiple of Oracle blocks
rem
rem Requirements: select on sys.dba_data_files
rem select on sys.dba_free_space
rem select on sys.v_$parameter
rem
rem --------------------------------------------------------------------

prompt Calculation of HighwaterMark of datafiles
prompt ---------------------------------------------------------------------------------------------------------

set feed off
set serveroutput on
execute dbms_output.enable(2000000);

declare
cursor c_dbfile is
select tablespace_name
,file_name
,file_id
,bytes
from sys.dba_data_files
where status !='INVALID'
order by tablespace_name,file_id;

cursor c_space(v_file_id in number) is
select block_id,blocks
from sys.dba_free_space
where file_id=v_file_id
order by block_id desc;

blocksize binary_integer;
filesize number;
extsize number;

begin

/* get the blocksize of the database, needed to calculate the startaddress */

select value
into blocksize
from v$parameter
where name = 'db_block_size';

/* retrieve all datafiles */

for c_rec1 in c_dbfile
loop
filesize := c_rec1.bytes;
<>
for c_rec2 in c_space(c_rec1.file_id)
loop
extsize := ((c_rec2.block_id - 1)*blocksize + c_rec2.blocks*blocksize);
if extsize = filesize
then
filesize := (c_rec2.block_id - 1)*blocksize;
else
/* in order to shrink the free space must be uptil end of file */
exit outer;
end if;
end loop outer;
if filesize = c_rec1.bytes
then
dbms_output.put_line('Tablespace: '
||' '||c_rec1.tablespace_name||' Datafile: '||c_rec1.file_name);
dbms_output.put_line('Can not be resized, no free space at end of file.')
;
dbms_output.put_line('.');
else
if filesize < 2*blocksize
then
dbms_output.put_line('Tablespace: '
||' '||c_rec1.tablespace_name||' Datafile: '||c_rec1.file_name);
dbms_output.put_line('Can be resized uptil: '||2*blocksize
||' Bytes, Actual size: '||c_rec1.bytes||' Bytes');
dbms_output.put_line('.');
else
dbms_output.put_line('Tablespace: '
||' '||c_rec1.tablespace_name||' Datafile: '||c_rec1.file_name);
dbms_output.put_line('Can be resized uptil: '||filesize
||' Bytes, Actual size: '||c_rec1.bytes);
dbms_output.put_line('.');
end if;
end if;
end loop;
end;
/

set feed on
prompt


dunno if this is what you are looking for