Error on boot

When I turn on my iMac I get the apple chime, the grey screen with the Apple logo then straight away I get text in the top right corner(see image) then it reboots... No keyboard shortcuts cmd opt r etc seem to work.. Could this be the hard disk?

G'day and welcome to the forums.

Course they won't work as you are experiencing what is known as a Kernel Panic attack.

Go into Console in Utilities and post the whole message here for assessment,.

I can't get into OS. It just shows that error and reboots and is just in an endless loop like that

Try booting in single user mode. Also what utilities or applications have you installed lately? If you are using OS X.7 or later try to boot into Recovery mode and see if any success. Of course you do have a current backup available?