If you want to install OSX on it, this is what I've done (without a USB or bootable drive). First, if you're replacing the hard drive with the SSD (for example), then you just need to plug in the harddrive you replaced and boot it up via USB. Format the new hard drive you installed as a Mac OS Journal (eg: a drive that Mac understands it can install an OS on). Then, restart the computer, press CMD+OPT+R to start up the internet recovery off of the USB-attached hard drive, which will allow you to install a copy of Lion or Mavericks or Yosemite (depends on what was on your old hard drive) to the new one within the computer.

So far, It's installing and it seems to be doing amazingly well.