It is not a good idea to change the default location of DP files, and, when you decide that it is time to update to DP 8, it is going to cause you all kinds of grief
In your case, all you can do is to stop DP, move all the files currently in /opt/omni to /omni/opt, create a symbolic link back to /opt/omni, and see if DP will restart. If disk space is the issue, you may want to create a new logical volume for /opt/omni
Other than keeping the same directory structure, I really don't see an upside to this, the /opt/omni and /etc/opt/omni directory is not very large. Sometimes with a very large IDB, it may be necessary to have /var/opt/omni in its own location