The Wikipedia article on Olga of Kiev claims, without any citation, that she was "hypothetically of Varangian extraction Old Norse: Helga." What evidence is there to support this? Moreover, is there any evidence that she was the daughter of Oleg of Novgorod?
Looking back through the history of the article I found some more details in a paragraph that has since been deleted by someone for being "a pack of nonsense from non-reliable sources":
The Life of Saint Princess Olga explains that she was born in the village Vybuty (Выбуты), Pskov Oblast, 12 km south of the city Pskov on the river Vilikoye, into a family of Varyag origin. By some accounts, she was the daughter of Oleg of Novgorod.
This cited Russiapedia:
According to the most traditional theory, recorded in the Primary Chronicle, Olga was born in Pskov (currently a city in the northwest of Russia) into a family of Varyag origin. [...] This theory about Olga's birth also explains the origin of her name, which is derived from the Scandinavian "Helga." Other historical versions state that Olga was either a daughter of Oleg Veshchy, the founder of the state of Kievan Rus, or had Bulgarian roots.
The Primary Chronicle doesn't actually appear to say that Olga was of Varangian origin, only that she "was brought from Pskov." The introduction to the translation by Cross and Sherbowitz-Wetzor (1953) concurs that Olga reflects the Old Norse Helga, and a note reads:
Hagiographical tradition characterized Olga as of Scandinavian origin and of non-noble birth; Makariy, Istoriya Russkoy Tserkvi, I, 3rd ed. (Spb., 1889), 268; Golubinsky, Ist. Russ. Tserkvi, I, i, 74.
If she was of "non-noble birth" then that would presumably rule out her being the daughter of Oleg, and I've found no sources for that one. However, I have neither book titled История русской церкви, and I can't read Russian anyway, so I'm not able to follow it up further.
Our lack of primary sources hampers us, as in most cases. I would probably fall back on a couple facts and then leave those up to interpretation. First, 'Varangian' was the Byzantine name for soldiers serving in their armies who originated from Northern Europe, specifically the Scandinavian Peninsula. This shifted in later times, as more Anglo-Saxons entered the Varangian guard after the Norman Conquest of England - that's another story. So stating that someone was 'Varangian' or 'Varyag' (an other form of the word Varangian) is perhaps inaccurate, as she would definitely have not served in the Guard. If the question is whether or not she was Scandinavian, the question becomes more broad.
Obviously as you know, there had been significant Scandinavian settlement and intervention in Eastern Europe in the Early Middle Ages, specifically starting with the viking Rurik and leading to the creation of the Kievan Rus. By Olga's time the Scandinavian influence was being interwoven with Eastern Slavic culture and peoples. Thus Olga's son, Sviatoslav, was a direct descendant of Rurik but had taken a Slavic name and utilized Slav soldiers in his army. Could Olga have been hypothetically Scandinavian? It seems likely. The name Olga does seem to be derived from the Scandinavian Helga, as you've already mentioned. Could she have been a local Slav woman who had been given a Scandinavian-rooted name? Absolutely.
I think the crucial facts are that during her son's minority she gained quite a lot of power and wielded it effectively. I'm actually not completely sure if the Scandinavian elite still had a hold on power in the Kievan Rus, but if they did then it would have probably been harder for a non-scandinavian woman to wield power I would expect. The other thing to consider is whether or not Igor (Olga's Husband) would have married a non-scandinavian woman. Perhaps so, but I don't know enough about the period to say definitively what the custom was.
Essentially what I'm saying is that the 'Varangian' or Scandinavian extraction theory of Olga's origin is not impossible, but as you've already explained there's not a ton of evidence either way. We do have evidence of extensive Scandinavian settlement in the Kievan Rus (they basically founded it), and it is more than possible that Olga was descended from these people. I know that wasn't near as definitive as you are probably hoping for, so I apologize. Hopefully someone more learned than I can help out too!
Sources: For a general overview of the period (and you seem to have a pretty good grasp already, expecially as you're citing primary sources!) you might wanna take a look at this article that focuses on the origins of the Eastern European cultures and identities.