OPL, while politically correct, seems redundant. All librarians are just one person. So I prefer the term SOLO librarian. It is also the name chosen by the largest group of SOLOs in the world, the SOLO Librarians Division of the Special Libraries Association, with over 1000 members. You won't find a SOLO in most public libraries, nor in college or university libraries.
Most SOLOs work in special libraries.
A special library is one serving a specialized or limited clientele, with specialized or limited materials and services. The emphasis is often on providing information, rather than books. The collection is often small. The librarian nearly always reports to a non-librarian and the library is part of but not considered critical to the parent organization's main mission.