When the free disk space level starts getting lower and lower, it is time to add disk(s) to feed your database. Most likely and advisable, your database is running on ASM (Automatic Storage Management) which provides abstraction over physical disks perfectly. Here are the steps of adding new disks:

1. Add new disks to your server physically. Assuming your database is running on a server class system or as a virtual machine,  it is possible to add disks without shutting down the server through SCSI interface either way.

2. Force Linux to discover new disks without need to reboot:
$ echo "- - -" > /sys/class/scsi_host/host0/scan

Run the commands below to check. You should see new disks and log entries regarding them:
$ fdisk -l
$ ls /dev/sd*
$ tail -f /var/log/message

3. Create a partition on each newly discovered disks:
$ fdisk /dev/sdx
Press 'p' to list partitions and see there is none. Then 'n' to create a new one. Answer questions as p, 1, default, default respectively. Press 'w' to save configuration.

4. Mark each disk as ASM disks:
$ /etc/init.d/oracleasm createdisk <disk_name> /dev/sdx1
Marking disk "<disk_name>" as an ASM disk:                   [  OK  ]

5. To add disks to a ASM disk group, as oracle run asmca tool.

Right click the disk group to be expanded and select "Add Disks"

Select disk(s) to add from the eligible disks list and click OK. When you return the first screen notice that free space is increased.