Right now, federal law and court precedence have twisted the 14th Amendment to mean that even the children of illegal aliens are citizens if they are born on U.S. soil. So the question of whether or not you need only one parent to be a citizen or both parents to be citizens at your birth has been answered differently.
If Obama was born on US soil to Stanley Ann, and if Obama Sr. was indeed his birth father, he's a US Citizen.
He is NOT, however, a Natural Born (US) Citizen, per the Framers' understanding of that term. He would need to have been born to two US citizen parents on US soil to meet the definition.