OSL is also the IATA metropolitan city code for Oslo, basically meaning "all airports around Oslo". Some booking engines support these for convenience - for example, ones using Sabre as a backend may support QSF (also listed in this file) for all airports around San Francisco. I flew to Oslo recently and know I saw OSL used as a metro code first on at least one booking engine. It doesn't really make sense to put these in a list of physically visited airports but I wouldn't expect most people to know the intricacies of IATA codes.

Why it's designed this way with namespace collisions, I have no idea. Technically QSF also is a tiny airport in Algeria.