What do the cryptic letters "o.m.p." mean for a Canon lens? For example, EF 24-105mm f/4L IS USM O.M.P."
It is not really a Canon term as much as it is an abbreviation often used on lists of used photo gear. It is shorthand for "Original Manufacturer Packaging" indicating everything in the original box when new is still with the lens.
Depending on the exact piece of equipment it could include such items as: The box itself, any instruction manuals or guides, software discs, internal plastic wrappers and other packing materials, lens caps & covers, hoods, straps, battery chargers, battery, AV cables, etc.