Oracle Database Discussions

We are pretty novice with Oracle DB and recently had to work with Oracle DB as some of our customers require Oracle. We have a system where we frequently delete all tables and recreate them with refreshed data provided to us via flat files. And we do not have any flexibility to identify the delta so we have to wipe off the whole data and reload it. We have been successful at it while using MSSQL and MySQL. But now that we are working with Oracle we see that when we delete the tables and recreate them apparently the space used by older tables is not being released, and tables are recreated taking new space.
We looked around for it and read about recycle bin. We purged recycle bin but see no effect on space used. It still the same. And now the space used by database stands at 29GB, as reported by Enterprise Manager, Main Page, Space summery.
We have imported the same into MySQL and see the total space utilized does not exceed 12GB including indexes.
So essentially the question is how can we determine the disk space used by our database? And if our tables are using space as expected then how to determine what is using rest of the space?

Thanks.

Talha.