Could anyone give the steps for converting a logical volume to a raw disk to mount Oracle ASM Instance on it ?

Please change your title to something descriptive---"urgent" will only annoy people. Some will not evern respond to a thread with that kind of title.

I assume you are talking about LVM---isn't a logical volume just a pseudo-device which then maps to one (or more) physical devices? I'm assuming that you can simply delete the logical volume, but of course it would delete all the data there also.

Before you do all of this I suggest you perform a full backup.

Basically the high-level procedure will be something like this:

1.) Move the data off the disk that you wish to remove from the volume group (utility is pvmove, use the manpages).
2.) Remove the logical volume from the volume group using 'vgreduce'. This could be difficult if you don't have enough available disk space within the volume group.
3.) Run 'pvremove' to unlabel the disk.
4.) Edit /etc/sysconfig/rawdevices - Add something like - '/dev/raw/raw1 /dev/sda1' depending upon your device names.
5.) 'service rawdevices restart'

Remember that Oracle requires specific permissions for both voting and OCR disks, so you should keep this in mind as well.

Remember that rawdevices are being deprecated by Red Hat, so if this is for anything other than testing you should use block devices mounted with O_DIRECT instead.