Correct

If some kind of process has opened a file, it still possible for you to delete the file at the command line.

Visually it will look as if the file has been removed.

However, underneath the file is still there. Only when the process, which opened the file, closes the file, the file will be really removed and the data blocks which made up the file will be free'd.

As BOFH already mentioned the most likely candidate for who has the files opened is one of the Oracle processes.

So you indeed have to shut down Oracle to get the space back which was taken by the file.

Furthermore, I hope for you you don't end up with a totally damaged and corrupt database.

It is not smart to remove files like that. There is a good chance you removed part if not all of the database.