I enjoyed the business letter page. https://www.usingenglish.com/resourc...r-writing.html
One of my coworkers signs letters with "op" in the area where I'd expect something like "pp" (which I had not heard of until this page. thanks!).
So I wondered if that's a valid but uncommon abbreviation for something meaningful. It is definitely not her initials.
I have never met it - perhaps they misread a poorly handwritten "pp" in the first place and have never asked what it means.