Esta consulta permite ver el espacio libre y el ocupado en oracle.

SELECT
  df.tablespace_name "Tablespace",
  df.bytes / (1024 * 1024) "Tamaño (Mb)",
  SUM(fs.bytes) / (1024 * 1024) "Libre (Mb)",
  Nvl(Round(SUM(fs.bytes) * 100 / df.bytes),1) "% Libre",
  Round((df.bytes - SUM(fs.bytes)) * 100 / df.bytes) "% Usado"
FROM
  dba_free_space fs,
    (
      SELECT
        tablespace_name,SUM(bytes) bytes
      FROM
        dba_data_files
      GROUP BY
        tablespace_name
    ) df
WHERE
  fs.tablespace_name (+)  = df.tablespace_name
GROUP BY
  df.tablespace_name,df.bytes
UNION ALL
SELECT
  df.tablespace_name tspace,
  fs.bytes / (1024 * 1024),
  SUM(df.bytes_free) / (1024 * 1024),
  Nvl(Round((SUM(fs.bytes) - df.bytes_used) * 100 / fs.bytes), 1),
  Round((SUM(fs.bytes) - df.bytes_free) * 100 / fs.bytes)
FROM
  dba_temp_files fs,
    (
      SELECT
        tablespace_name,bytes_free,bytes_used
      FROM
        v$temp_space_header
      GROUP BY
        tablespace_name,bytes_free,bytes_used
    ) df
WHERE
  fs.tablespace_name (+)  = df.tablespace_name
GROUP BY
  df.tablespace_name,fs.bytes,df.bytes_free,df.bytes_used
ORDER BY
5 DESC;