How about dual-booting? A Macbook Pro will boot from an SD card or USB 3.0. You can happily run OS X from an SSD in a USB 3.0 enclosure. Just hold down the 'option' key during power-up.
As others have said, you could use virtualisation software. However, this will only work if you're happy installing that on your regular hard drive, and if you don't need hardware-accelerated graphics in an OS X guest.
As others have said, you could use virtualisation software. However, this will only work if you're happy installing that on your regular hard drive, and if you don't need hardware-accelerated graphics in an OS X guest.