There is only one file type that uses the .LO extension:
Interleaf Compiled Lisp File
Compiled Lisp (.LSP) file saved by Interleaf technical publishing software; contains compiled code that can be run within the program; often used for performing common or repetitive commands.
NOTE: Interleaf 6 and later (now QuickSilver) saves compiled Lisp files in the Fast Save format with an .FAS extension.
Updated: August 15, 2016