Casper Ruud's birth city (4)

I believe the answer is:

oslo

(Other definitions for oslo that I've seen before include "European capital city (found in Czechoslovakia!)" , "Solo in Norway?" , "It's capital in Anglos London" , "Norwegian chief port" , "European capital, known until 1924 as Christiania" .)

I've seen this clue in The Wall Street Journal.

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