HOS stands for ‘hardened oil syndrome’. For a period of time, Marklin used an oil that hardened which binds the gears. It does not permanently damage the locomotives, but the locos will not run, or be very sluggish if they have this syndrome. To correct the issue, the old oil will need to be cleaned out, the locos re-oiled. We like to be upfront about these locos. Typically, they can be purchased at a discount since maintenance will need to be performed.
The site http://www.zscale.org/articles/revival.html has informative articles on cleaning and servicing Marklin locomotives. Some locomotives can be brought back to life quite easily.