I see Private User and A. have been working in this area, and left notes in the About Me for Sigfried I von Oldenburg (b. 900).
I don't want to mess up their train of thought by adding, changing, or deleting information but it might be helpful to their discussion to point out the information given by James Anderson, Royal Genealogies (1732, 1736). This information is not reliable for modern genealogical standards, but it is a useful guide to what information was current in the 18th century. Earlier generations are highly likely to be fictitious.
According to the information on Table CXCVII, The old Counts of Oldenburg (p. 732) this family descends from Witekind. (Oh sure.)
1. Witekind the Great, Duke of Saxony, died 807.
2. Wigbert, Duke of Saxony, died 825.
3. Walpert, Count of Ringelheim and 1st Count of Oldenburg (he built the castle there), died 856; married Altburgis
4. Theodoric or Dietricus, Count of Ringelheim and Oldenburg, died 920; married Ludmilla or Hedwig (perhaps the Ludmilla deleted by Rod Hinckel)
5. Sigfridus, Count of Ringelheim and Oldenburg, 1st Margrave of Brandenburg, died 940 or 946; married Magdalene, daughter of Albert III, Count of Ascania
6. Ulric, Count of Oldenburg ("went into a Cloyster 938"), died 967; married Swana, daughter of Ratbod, King of Friesland [ Radbod, King of the Frisians ]
7. Otto I, Count of Oldenburg, fl. 982; married Michtild
8. John I, Count of Oldenburg, fl. 1007 and 1017; married Rixa or Adelaid
9. Huno the Glorious, Count of Oldenburg, etc.
There is more information there of one of you wants to create and disconnect a fictitious line here.
https://books.google.com/books?id=Ge9lAAAAcAAJ&pg=PR14&dq=%...
Medlands begins the Oldenburg line with that Huno, so he is probably the first reliably historical ancestor.
http://fmg.ac/Projects/MedLands/OLDENBURG.htm#_Toc418239652