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- Derrick or Dietrich of Oldenburg, Latin-based anglicization also Theoderic of Oldenburg (c. 1398-February 14, 1440), nicknamed Theoderic the Lucky or the Fortunate (Teudericus Fortunatus), was a feudal lord in northern Germany, holding the counties of Delmenhorst and Oldenburg. He was called "Fortunatus" as he was able to secure Delmenhorst for his branch of the Oldenburgs.
Dietrich is the father of Christian I of Denmark, a male-line ancestor to the present-day Danish throne under Margaret II of Denmark as well as the last czar of imperial Russia, Nicolas II.
Dietrich of Oldenburg was the son of Count Christian V of Oldenburg (who became count in about 1398 and died in 1423) and his wife, Countess Agnes of Honstein. His grandfather, Count Conrad I of Oldenburg (d. ca. 1368) had left his lands divided between his father and his uncle Conrad II.
Dietrich’s father, Christian V, managed to gain the upper hand when Conrad II's son Maurice IV died in 1420. After this, most of the Oldenburg family patrimony was under the power of Dietrich’s branch. However, the house had several minor branches who had estates and claims, as was usual in any medieval fief.
Dietrich of Oldenburg was the grandson of Ingeborg of Itzehoe, a Holstein princess who had married count Conrad I of Oldenburg. After the death in 1350 of her only brother, Count Gerhard V of Holstein-Itzehoe-Plön, Ingeborg and her issue were the heirs of her own grandmother Ingeborg of Sweden (d. ca. 1290, first wife of Gerhard II of Plön-Itzehoe), the eldest daughter of King Valdemar of Sweden and Queen Sophia, who herself was the eldest daughter of the sonless King Eric IV of Denmark and his wife Jutta of Saxony. Since other legitimate descent from King Valdemar apparently was extinct by this time, Dietrich was considered the heir general of Kings Valdemar I of Sweden and Eric IV of Denmark.
Dietrich succeeded his father as head of the House of Oldenburg in 1423.
He had firstly, as a child, married (for reasons of succession and uniting the hereditary fiefs) a distant cousin, Countess Adelheid of Oldenburg-Delmenhorst (who is said to have died already in 1404), daughter of Oldenburg Count Otto IV of Delmenhorst, and in 1423 he married for a second time, Helvig of Schauenburg (born in about 1398-1400, died 1436), widow of Prince Balthasar of Mecklenburg and daughter of the murdered Duke Gerhard VI of Schleswig-Holstein and his wife Elisabeth of Brunswick, thus sister of the reigning Duke Adolf VIII. All his legitimate children were born of the second wife.
His second marriage strengthened this interest in Scandinavian monarchies, since Helvig was a descendant of King Eric V of Denmark, King Haakon V of Norway and King Magnus I of Sweden.
At this time, all Scandinavia lived under the Kalmar Union erected by Queen Margaret I of Denmark. In 1387 she had lost her own heir Olav IV of Norway, the new heirs now being Eric of Pomerania, and his sister Catherine who was married with a prince of the Palatinate and Bavaria.
Count Theodoric of Oldenburg is said to have been a rival claimant to the crowns of Sweden and Denmark during the reign of Eric VII/ Eric XIII, whose succession was through Christopher I of Denmark, the younger brother of the murdered Eric IV, and through Magnus I of Sweden, younger brother of the deposed King Valdemar.
Count Theodoric had three surviving sons and one daughter:
Christian (1426-1481), who succeeded him as Count of Oldenburg and Delmenhorst, and became later King Christian I of Denmark, Norway and Sweden, as well as Duke of Schleswig and Holstein.
Maurice (1428-1464); when his elder brother became king, he was given the County of Delmenhorst.
Gerhard (1430-1500); when his eldest brother had become king, he was given the county of Oldenburg, and from his other brother's heirs he also inherited Delmenhorst in about 1483. The third son got his name from usages of the mother's Holstein clan.
Adelheid (1425-1475), first married count Ernest III of Hohnstein (d. 1454) and then in 1474 Count Gerhard VI of Mansfeld (d. 1492).
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Abt 1398 - 1440 (~ 42 years)
, d. 21 May 1481, Copenhagen, Kobenhavn, Denmark 
