The OnePlus One has not all reached the customers who are trying to get it. The pre orders begin and go down. That does not mean that the company is not working on the successor of the device.
According to a report the OnePlus Two is all set to launch in the third quarter of the coming year. We had rumors any this time frame earlier too, and now it seems that they might turn out to be true. However the exact time frame is still not known yet, Q3 would end in the month of September. So we would be seeing the deduce close to that month. And also OnePlus recently made its way to India.
And the next generation would come to India pretty soon and India would be asking the first markets to receive the device. Till now we know nothing about the hardware and the specifications of the OnePlus Two, but some rumors suggest that it would be smaller than its predecessor.