Huh? With the presidency it depends on the exact location of his birth and whether having only one citizen parent makes prevents him from being a "natural born" citizen. But only the president and vice-president have that requirement - not the Senate. Unless he gave up his citizenship somehow, I don't see how he could be ineligible.
Here are the requirements to serve in the US Senate:
You must be:
- At least 30 years of age
- A citizen of the US for at least 9 years
- A resident of the state you’re representing
Therefore, if Obama had been a US citizen for at least 9 years prior to his US Senate run, you’d be absolutely correct.