Sure, but NYC isn't also a specific New York City airport (JFK, EWR, LGA). The problem here is that OSL is apparently used both for the metropolitan area and for one specific airport (in the metropolitan area).
I guess you missed the second half of the sentence you quoted. Where I note that Houston, the city, is HOU; and the Houston airports include IAH, EFD, and HOU.
Sure, but NYC isn't also a specific New York City airport (JFK, EWR, LGA). The problem here is that OSL is apparently used both for the metropolitan area and for one specific airport (in the metropolitan area).