Android 7.0 is being tested on the OnePlus One, but it's not an official version
It seems that the OnePlus One was spotted on GFXBench, and Geekbench, both showing Android 7.0 running the show. This got several people excited, thinking that an update to Nougat was being prepped. But if you still use the OnePlus One, don't get worked up. The version of Android 7.0 being tested on the phone is not an official release. In fact, it is a third party ROM that we told you about last August. Taken from the AOSP source by XDA member updateing, almost everything was working on the ROM back then except for the video camera and the stock keyboard.
The OnePlus One is not going to be officially updated past the current Marshmallow build. If you want to run Android 7.0 on the phone, you will have to rely on the work of a third party developer like updateing.
source: Geekbench via Telefoonabonnement.nl (translated)