Yet, Obama strikes me as the kind of guy who would remember what kind of grades he got, and would think it a perfectly reasonable question.
Thus Maraniss interviewed hundreds of people (and got an Oval Office interview, in which Obama volunteered that he was a B+ student at Occidental and an A- student at Columbia).
On p. 385 of the incredibly detailed (and dull) Barack Obama: The Story, Maraniss says:
"Two years at Oxy (during which his cumulative grades were 'a B-plus,' he later said) ..."
And on p. 465 Maraniss writes:
"He had received mostly A's in his coursework during those two years [at Columbia], he said later, and finished with a 3.7 grade point average."
But, I'm just making that up. Who knows? (Until you ask him.)
As for the question of whether Obama benefited from affirmative action, well, he wrote that "undoubtedly" he did. As Obama wrote when he was at Harvard Law:
I must say, however, that as someone who has undoubtedly benefited from affirmative action programs during my academic career, and as someone who may have benefited from the Law Review’s affirmative action policy when I was selected to join the Review last year, I have not felt stigmatized within the broader law school community or as a staff member of the Review.
On the other hand, Harvard Law let Michelle in, and Barack's obviously a lot smarter than his wife. So, he probably didn't need as much racial preference as some of the other black students.