Norway's King Olav V, 86, the world's oldest reigning monarch, died yesterday in Oslo after a heart attack. The king, who had a stroke in June, was a British-born Dane who became second monarch of modern Norway in 1957. His ancestry was firmly rooted among the blue bloods of Europe, but the Norwegians called him "Folkekongen" -- the people's king. The only child of a Dane who became the Norwegian king after Norway's union with Sweden was dissolved, he was groomed for the throne from the time he was 2. He became a rallying point for his adopted countrymen during World War II.