OnePlus One released a video of their own quality test. The phone went through a several tests like pressure test, drop test, heat and weather test and so.
OnePlus One was pressed with 25kg (55 pounds) thousand times. This was done to test the pressure test. Later it went through drop tests, where it was dropped 40,000 times, a tumble tests with 150 cycles spin and an impact test with 1.2m meter drop.
The video also has port tests, such as USB stress test and port durability tests. It was tested for ports various mishandling types.
It also went through weather test, where it was exposed to rain for ten minutes and heat test for 4 hours in 55° Celsius heat.
The video clearly shows the OnePlus One had a tough testing procedure. It was meant to show the buyers that OnePlus One is a durable phone.
Source: Android Authority