No, you don't lose your US citizenship when you become OCI. For what it is worth, US State Department also agrees that OCI is not dual citizenship, but something akin to the US Green Card.
From
http://travel.state.gov/travel/cis_p.../cis_1139.html
SPECIAL CIRCUMSTANCES:
Dual Nationality: In 2006, India launched the "Overseas Citizens of India" (OCI) program, which has often been mischaracterized as a dual nationality program, as it does not grant Indian citizenship. Thus, an American who obtains an OCI card is not a citizen of India and remains a citizen of the United States. An OCI card in reality is similar to a U.S. "green card" in that a holder can travel to and from India indefinitely, work in India, study in India, and own property in India (except for certain agricultural and plantation properties). An OCI holder, however, does not receive an Indian passport, cannot vote in Indian elections and is not eligible for Indian government employment. The OCI program is similar to the Persons of Indian Origin (PIO) card introduced by the Indian government several years ago, except that PIO holders must still register with Indian immigration authorities, and PIO cards are not issued for an indefinite period. American citizens of Indian descent can apply for PIO or OCI cards at the Indian Embassy in Washington, or at the Indian Consulates in Chicago, New York, San Francisco and Houston. Inside India, American citizens can apply at the nearest FRRO office (please see “Entry/Exit Requirements” section above for more information on the FRRO). For more information on the OCI program, please see
http://mha.nic.in/uniquepage.asp?Id_Pk=551.