DOS XL is a discontinued Disk Operating System (DOS) written by Paul Laughton, Mark Rose, Bill Wilkinson, and Mike Peters and published by Optimized Systems Software (OSS) for the Atari 8-bit computers.

A direct descendant of OS/A+, DOS XL provided additional features to Atari's equipped with floppy disk drives.

These included single and double density support, a command-line mode (called the command processor or CP), a menu mode (an executable loaded in memory), batch file support, and support for XL extended memory and OSS SuperCartridge banked memory.

As more features and add-ons were included, these were placed on the double-density side only due to lack of disk space.

This was due to OSS's own experience that resulted in faster writes with virtually no risk of errors.