OS's Are never pre-installed on any virgin drive.
Many drive makers format external disks, but rarely do on internal drives. If they do its for MS Windows, not Mac OS-X, and you'll need to reformat the drive before you can use it.
OK, you'll need to get an external case to house your SSD or prep up a USB thumb drive as a bootable drive. I would recommend the USB thumb drive as you can verify it is working before opening the system and besides setting up a drive to deal with mishaps is a good idea! You still want to get the external case to repurpose your old SSD and besides you may want to move some old stuff over to your new bigger SSD.
Download if you haven't already the OS installer from the App store. as an example here's a link to the current version: OS-X Yosemite.
Once you have downloaded it make a copy so you have a spare copy (placed in a different folder and/or drive).
As to getting an external case to house your SSD this is what we use: OWC Envoy. With the case you can use your old SSD as a backup drive.