Three minutes [after liftoff], during the burning of the third stage, the payload fairing — a clamshell nose cone that protects the satellite as it rises through the atmosphere — failed to separate as commanded.
....“The fairing has considerable weight relative to the portion of the vehicle that’s flying,” said John Brunschwyler, manager of the Taurus rocket program for Orbital Sciences of Virginia, which built both the rocket and the satellite.
“So when it separates off, you get a jump in acceleration,” said Mr. Brunschwyler. “We did not have that jump in acceleration. As a direct result of carrying that extra weight, we could not make orbit.” The satellite fell back to Earth, landing in the ocean just short of Antarctica.
More here from Jonathan Hiskes at Gristmill about what the OCO was supposed to do.