The law at the time said that: 1) if he was born here, he is an American citizen; 2) if he was born outside the country, then his parents had to be citizens residing in the United States for at least 5 years after age 14 prior to his birth. His mother did not meet the age requirement to confer citizenship on an international birth, and his alleged father was a Kenyan citizen.
What is open for debate is whether Obama Sr. is his father, as well as where Obama was born. If Obama Sr. was covering for someone else impregnating his mother, then he may very well be a citizen.
The final issue for debate is what constitutes "natural born" citizenship as a Constitutional qualification for President, aside from a general class of citizenship. Writings from the generation of the Founders is mixed on this.
-PJ
I think you're saying if he was born to at least one parent who was a U.S. citizen he would be considered natural born. I also believe that to be the case. But there seems to be disagreement about that.