LO is the extension given to files related to the multi-paradigm programming language called Lisp. LO files are used to store a compiled version of a program written in Lisp. They are based on LSP files. LO files are saved in Interleaf format. Such files are used by a specialized computer publishing software package called Interleaf. They allow you to automate some tasks in the program, e.g. frequently repeated activities.

Interleaf is a desktop publishing tool developed by BroadVision.