Reduce OS imaging time using 'just in time' file delivery

Embodiments are provided for imaging an operating system (OS) by creating a new OS image from an installer OS image copy maintained in durable storage. During the OS imaging, only a subset of OS files from the installer image are fully copied into the new OS image. Placeholder files are created for other files not included in the initial subset of OS files which were determined to be critical for booting of the OS and/or a minimum set of OS functionality. The placeholder files are distinguished from sparse files and are inaccurately presented by the file system as being full copies of the underlying installer OS image. The data for the placeholder files is only copied when requested, on demand, and/or when there is available/unused processing bandwidth that is subsequently identified after rebooting the computing system with the new OS image.