Each Droplet is akin to a blank canvas. You'll choose the Operating System the Droplet is deployed via the DigitalOcean Control Panel after you click on "Create Droplet."
It's the first option and you can choose Ubuntu, Debian, CentOS, FreeBSD, Fedora, or CoreOS.
Once the Droplet is live, the OS you chose from the list is already installed and ready to go, though you'd need to install the software needed to run specific applications as in its current state, the only access to the Droplet that you have is Console or Terminal/PuTTy -- i.e. the CLI.
You can install a graphical desktop as well as multiple versions of Python. These can be done after the Droplet is live.