http://en.wikipedia.org/wiki/Comparison_of_boot_loaders
type `grub boot loader` into youtube....
hve to admit i dont know alot aboutt he osmc.
You could attach an external hard drive via the usb, and stick other opsys on that. Maybe change it so that grub loader boots from the sd card and then you select the os you want that is on the external hdd.....which i assume you can get grub to see.
you can buy sdcards off ebay that have an os preloaded on hem and setup to work on rasppi......just swop cards to boot a differnt os.....keep it simple)