[Kernel Build] Initrd not getting build out of source dir

Hi Kernel Alias,

I downloaded latest kernel source from kernel.org and tried to build it
at a directory outside source by giving O=<path option> but I noticed that
initrd image is not getting build but if I execute make inside source
things are fine.

[Build Kernel/Modules etc inside source]
~/linux-stable/> make
~/linux-stable/> make install
~/linux-stable/> make install_modules
~/linux-stable/> su -c "reboot -f"

everything works out fine and system gives me selection to boot from
newly build kernel and system boots up without issues. But if I build as
below I see system OOPS because no file system found for mount. Would
like to know why?

[Build outside]
~/linux-stable/> make O=~/linux-build/
~/linux-stable/> cd ../linux-build/
~/linux-stable/> make install
~/linux-stable/> make install_modules
~/linux-stable/> su -c "reboot -f"

Regards,
Vipul.