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.

I had the same thought. I’ve seen some booking sites default to “OSLALL” when you select Oslo, which will show results for both OSL and TRF. Probably “OSLALL” is not a real IATA. Either way, to me OSL should mean Gardermoen and nothing else. And search results I am finding on Google seem to indicate the same. After all, it’d be pretty silly if you were traveling to Gardermoen and your luggage was tagged “OSL” but then your luggage somehow ended up on TRF instead.

So referring to OSL as “all airports around Oslo, Norway” is IMO a mistake on the part of the OpenBSD developers indeed. In spite of the point made by the sibling comment about ambiguity. Because the context is airports, I think OpenBSD should list OSL as “Oslo Gardermoen Airport, Norway” in their airports file.

See also https://en.wikipedia.org/wiki/Oslo_Airport,_Gardermoen which says:

> Oslo Airport (Norwegian: Oslo lufthavn; IATA: OSL, ICAO: ENGM), alternatively referred to as Oslo Gardermoen Airport or simply Gardermoen