Complex sentence: Oscar Bernadotte renounced his titles as "Prince of Sweden" and "Duke of Gotland" when he married morganatically in 1888 but was allowed by his father King Oscar II to keep the courtesy title of Prince and then be styled "Prince Bernadotte".
Split into simpler sentences: Oscar Bernadotte renounced his titles as "Prince of Sweden" and "Duke of Gotland" when he married morganatically in 1888. However, he was allowed by his father, King Oscar II, to keep the courtesy title of Prince and then be styled as "Prince Bernadotte".