Selberg's advisor? Does anybody know who was Atle Selberg's advisor?
I find it interesting to know the advisor's impact on his students.
Unfortunately, in Selberg case, this information (even his advisor's name) seems to be nowhere to be found.
 A: It appears that no single mathematician at Oslo had that much influence on Selberg. There is a long translated interview.
He defended his dissertation on October 22, 1943. The Germans closed the university on November 30. Given the kind of people with whom he was corresponding and the comparatively little time he actually spent at Oslo, it may be that the professors flipped a coin after his defense as to the order of signatures. 
Well, this may be an injustice to someone we currently do not know about, but Selberg seems to have done everything himself.
There may be reason to think Carl Størmer (1874–-1957) was the adviser. He was present at the defense and presented the paper to the Norwegian academy of Sciences. Evidently the expert asked to comment was elsewhere, Harald Bohr (1887-–1951), brother of Niels.
A: I could assume that Fritz David Carlson (1888–-1952) was at least his informal advisor, although Carlson was a Swedish mathematician. Note that he is not represented in the Mathematics Genealogy Project.
Selberg's first contribution to the Polya-Gelfond problem was very close in style to Carlson's famous theorem (see, for example, 
[P.J. Forrester and S.O. Warnaar, The importance of the Selberg integral, Trans. Amer. Math. Soc. 45 (2008) 489--534]).
EDIT. This was only my guess (as I indicate in my comments above to the question, I have believed that Selberg was self-educated). I asked the question Gert Almkvist, who knows a lot of Scandinavian math history. He answers:

It is evident that Selberg had no
  advisor, but somebody had to sign the
  papers. Harald Bohr fled to Sweden
  when the Germans invaded Denmark. He
  was in Lund and Stockholm. But he did
  not dare to go to Oslo for the thesis
  defense.

Then Gert adds that he has a huge biography of Harald Bohr but this could serve for a different question. $\ddot\smile$
A: I'm a student at the university of Oslo, so I thought I'd have a go at this. I just talked to Erling Størmer (Carl's grandson) who is a professor emeritus here. He said that in practice Atle had no advisor. Of course someone must have signed the papers but he doesn't know who (I don't really see what difference it makes anyway). Erling told me that according to Atle the reason why so many Norwegian mathematicians at the time worked in number theory is that they were all self-taught, and number theory is more accessible to the autodidact.
A: Here is a quote from the translated interview, with a little emphasis from me:  "It was Størmer that presented it to the Norwegian Academy of Science and Letters in Oslo, of course.  As opponent Harald Bohr was the obvious choice because there was nobody in Norway that had any real competence in that field."  Elsewhere in the interview, Selberg says, "There were others that were helpful in their way, my brother Henrik and Professor Størmer, in particular."   He also talks about reading Størmer's lecture notes when he was in grade school, and meeting him when he first came to Oslo as an undergraduate.
It is not clear whether the University of Oslo formally required an "advisor".  But we can conclude this from the interview:  On the one hand, Størmer was Selberg's undergraduate mentor and the communicator of his PhD thesis.  On the other hand, by the time Selberg filed his PhD thesis, he was really an independent mathematician with several great publications, with no one else working in his field in Norway.  Selberg is not very charitable on this point.  The arrangement with Bohr was more striking than Will's comment suggests.  Bohr would ideally have come to Oslo for the thesis defense, but since he couldn't, he received Selberg's report in absentia in Sweden, and "Størmer read Harald Bohr's report."  It's clear enough that Bohr wasn't Selberg's advisor either.  Selberg generally has more to say in the interview about what he read than who he met.
The other guy on Selberg's committee was the Skolem, who as Selberg points out worked in logic rather than number theory.  In fact, I only found evidence of one other well-known mathematician in Oslo in that period, Heegaard.  But Heegaard retired in 1941, and decades earlier he had left Copenhagen in a huff because they had hired Harald Bohr!
So probably the right answer is that Størmer was Selberg's nominal advisor, if he had one, but he was actually self-advised as a graduate student.  Selberg gives little room for other possibilities in the interview.
