Duke Constantine Frederick Peter of Oldenburg was a Duke of the House of Oldenburg. He was the grandfather of Duke Peter of Oldenburg as well as grandfather of Grand Duke Nicholas Nikolaevich, General of the Imperial Russian Army during World War I. His great-great-grandson, Nicholas Romanov, was the President of the Romanov Family Association until his death in 2014.

Duke Alexander Frederick Constantin of Oldenburg was the second son of Duke Peter of Oldenburg and his wife, Princess Therese of Nassau-Weilburg. Though he had a German title and ancestry, Alexander and his siblings were born and raised in St. Petersburg as the grandchildren of Grand Duchess Catherine Pavlovna of Russia.