It’s been posted here numerous times.
3. January 13, 1941 to December 23, 1952
If you were born between January 13, 1941 and December 23, 1952, you automatically acquired U.S. citizenship if both your parents were U.S. citizens and at least one had a prior residence in the United States. You didn’t have to do anything special to keep your U.S. citizenship.
If only one parent was a U.S. citizen, that parent must have lived in the United States for at least ten years prior to your birth, and at least five of those years must have been after your parent reached the age of 16. To keep your citizenship, you must have lived in the United States for at least two years between the ages of 14 and 28 (called a residence requirement). However, as a result of a U.S. Supreme Court decision, if you were born after October 9, 1952, your parent still had to fulfill the residence requirement in order to pass citizenship to you, but your own residence requirement for retaining U.S. citizenship were abolished. If your one U.S. citizen parent was your father and you were born outside of marriage, the same rules applied if your father legally legitimated you before your 21st birthday and you were unmarried at the time.
I've heard people imply that these laws hold even if Obama was born in the United States—that particular claim (which you may not be making) is what I'm looking for info on.