I think Obama was probably born in the US. O'Reilly had his investigators look back at 2 news papers and they found birth announcements in the paper archives. I know this can also be forged, but don't believe it.
I also think a lot of this was brought on himself. There was a time, probably still true, he wasn't proud to be a US citizen and it was in his interest, for book selling, etc., to not be a US citizen. That's why he told his publisher he was born in Kenya.