The testing was made up of taking the phone out in the rain, and then dropping it inside a few puddles for as long as ten seconds. In each case, the OnePlus One lived through the dunking, although without a certification rating, there is no telling how much water the flagship handset can take. According to one member of the OnePlus team, getting the phone certified would have cost too much money, driving up the price of the handset in the short term.
The problem is, without the certification number, who really knows how much the OnePlus One must be protected from the elements. Even testing the phone by putting it inside a puddle for ten seconds, doesn't mean that it will always be protected from water. If you do own the phone, or plan to, use common sense and try to keep your phone dry.
The video, by the way, was filmed using the HTC One (M8), which is IPx3 rated. That means that the phone is protected from water spraying up to 60 degrees from vertical at 10 liters/min at a pressure of 80-100 kN/m2 for 5 min. In other words, taking your HTC One (M8) out in a light rain, like what we witnessed in the video, is okay.
Thanks for the tip!