Does oracle release space to operational system when is issued
alter table ... shrink space (one step) or just after move HWM (two steps)
alter table ... shrink space compact?
Table Shrinking is not about freeing space for OS, but replacing the blocks freed due to
DELETE operations, with the used blocks. Thus, reducing the load on the database during table scans. Moving HWM does not 'give back' the free blocks to OS, it just indicates where the used blocks end, therefore Oracle knows there is no data after that.
Managing OS space for a database is done in OS layer, if you want to free up some space in OS, and you know that there are some free blocks inside Oracle datafiles, you have to take a dump using Oracle Datapump (
impdp), drop the datafile, re-create the datafile with
AUTOEXTEND option and then import your dump.