Yes, it is possible to choose a specific OS as a resource in SkyPilot. By default, SkyPilot uses Ubuntu as the OS. However, if you want to use CentOS 7, you can choose a custom CentOS 7 AMI. When launching your job, make sure to have the necessary tools installed in the CentOS AMI (e.g., wget, conda, pip3, sudo permission). You don't need to create the user 'ubuntu' in your CentOS AMI.