Do not consider this to be legal advice.
I'd be careful with this one. Dangerous pair of two-letter abbreviations here, given the fact that an arbitration panel could easily construe the letters "OS" as having a correlation to the letters "HP" to mean "Hewlett Packard Operating System".
I will also add that HP is currently using the term "HPOS" to mean "Hewlett Packard Output Server", as you will see on this page:
http://www.hp.com/united-states/outputmanagement/support_resources.html
It would be a very bad idea to contact HP and try to sell it to them. Your best bet for present use would be a minisite which makes no mention of anything computer or technology related. Try to make it look like just a generic combination of letters and nothing more.
As for the valuation, they are very good letters, all premium, and would think you should have a relatively easy time landing low-mid to mid $xxx for it.