I had this problem and found out that the oracle was using 100% of the disk space because of the archive logs.
You can remove all archive logs (make sure you have backups in case you need to restore something later in case of a failed backup or crash) from /oracle/<sid>/oraarch.
I used "rm -R *" in this folder.
The oracle system freezes in this situation but the db is still up. When you remove the files, it takes a while but the execution later continues by itself (give it like 5 minutes and double check).