How do we free up disk space from allocated namespace? / ORA-03297

Question

We are running out of disk space.

The server has a 420 GB HDD, and runs a Microsoft SQL Server and an Oracle Server(10g).

The Oracle server uses up to 315 GB space. We have just loaded a 100GB Microsoft SQL Server database. Now we have approximately 5GB free and need to make a backup of the SQL Server database.

First we tried to move the backup to an other drive but this didn't work.

Then we tried to resize the allocated space of our Oracle Server, which consists of 6 files of 20GB each with about 4GB used in each file. When we tried to resize this file to 10GB, we got the Oracle Error ORA-03297 but it shows 4GB used. After several tries we could shorten some files to 16/17/18GB, but we are still in need of more space.

Is there a Solution how we can resize the Oracle files to 10GB each?

We've tried Coalesce and deallocate unused space, however that did not work either.