I had no idea Oldenburg had a family branch of German blood. I've always thought of The House of Oldenburg as the ruling house of Greece.
The Oldenburgs are a german dynasty!!
To insist that the Oldenburgs were exlusively German is not quite as absurd as thinking they were exlusively Greek, but lets not forget that they spread out over all of Northern Europe (Denmark-Norway, Sweden and Russia).
After the dynasty died out, the King of Denmark and the Duke of Schleswig-Holstein-Gottorp took over Oldenburg by heritage in 1667. In 1676 the King of Denmark was the only ruler on Oldenburg (I don´t know why S-H-G lost their rights)
Pure "might is right". That Duke Christian Albrecht of S-H-Gottorp did not receive his due share of Oldenburg-Delmenhorst from his father-in-law (and Head of the House of Oldenburg) King Frederik III was a major reason for the Gottorps becoming hostile to Denmark and forming alliances with Sweden. A technical reason was that the Duke of Schleswig-Holstein-Sonderburg-Plön for some reason also was entitled to some part of the inheritance. Being an ally of Frederik III, the Duke of Plön ceded his rights to Denmark, who thus controlled "the majority" of lots.
BTW Frederik III's natural son Ulrik Frederik Gyldenløve, Count of Laurvig, was married to Countess Antoinette Augusta von Aldenburg, the daughter of the natural son of the last Count of Oldenburg, Anton Günther II. Perhaps that alliance of bastard lines also played a role, considering that this natural son, Count Anton I of Aldenburg, became the first Danish Governor in Oldenburg!
BTW the whole fascinating line of descent Oldenburg-Aldenburg-Gyldenløve-Bentinck has been one long line of succession disputes!
He was married with the german Princess Sophie Auguste von Anhalt-Zerbst-Dornburg, who later became Empress Catherine the Great.
And who strangely enough inherited the Lordship of Jever, the former Oldenburgian neighbour fief of the Aldenburgs' County, the Lordships of Varel and Kniephausen, by virtue of descent from an Oldenburgian ancestress who had married into the House of Anhalt-Zerbst. Her grandson Emperor Alexander I ceded it back to Oldenburg in 1818.
In 1773 King Christian VII gave Oldenburg and Delmenhorst to Grand Duke Pavel Piotrvich (Paul I., son of Peter III. and Catherine II.), who gave the lands to the Gottorp Prince Friedrich August later that year. Friedrich August united both territories: Duchy of Holstein-Oldenburg.
It is more accurate to say that by this Treaty of Tsarskoe Selo, the Gottorp branch ceded the ducal Gottorp parts of Holstein to Denmark (but retained the Prince-Bishopric of Lübeck) and received Oldenburg and Delmenhorst in exchange, which the senior Russian branch in its turned ceded (together with Lübeck) to its junior branch.