I can only speak from my 40 years experience in Budo. I started in the early 1970's with JKA style Shotokan Karate.. The expression OSS was, and still is, used to mean a number of things like, I understand, yes, hello, goodbye, Karateka use the term to end letters, emails, phone calls etc.
It is much like the motto of the Irish regiments that use Faugh a Ballagh (clear the way). current serving and veteran soldies will use that term to end letters, emails etc. Manny time using FaB instead of writing it iut. The Irish Guards use their motto Quis Saparabit (who will seperate us ?) in much the same way. Often just written as QS.
Most people know the Irish origin of the American military regiment using the motto Garryowen..
In my Aikido training oss was never used. It is not used in Kendo or in any koryu that I know of.
There is no harm in using the term as long as you use it in the spirit of the art.