The 2014 flagship killer, OnePlus One, today appeared on the GeekBench website and surprisingly the device was running on Android 7.0 Nougat. OnePlus One was launched almost two and a half years ago and normally a device is only supported for Android updates up to a maximum of 24 months.
Further, OnePlus One runs on Snapdragon 801 SoC which, at the time when Nougat was released, was announced to be non-compatible with Android 7.0 as it doesn’t support Vulcan graphics API Google included in Nougat. However, it was recently rumoured that Qualcomm has released the new graphics drivers to support the API which is when we saw a bunch of Nougat custom ROMs for OnePlus One appeared on XDA. 
To spoil the party, the OnePlus One which was spotted on Geekbench might be running on a custom ROM. As we all know XDA community has a large number of developers who develop these custom ROMs on daily basis. So, chances are that one of the developers uploaded the same to test out the efficiency and performance on the bench-marking website.
In contrary, there is a possibility that Android Nougat is being tested on the original OnePlus One by the company itself. So OnePlus One users have to wait to get their fortune sorted.
However, the point here to note is that while the third-party developers are able to build an Android 7.0 ROM for OnePlus One, OEMs such as Samsung, HTC, LG etc doesn’t support their device for more than 24 months. This is because these companies want their users to move on to newer devices.
Nevertheless, OnePlus 3 users can get excited as the company is already working on Android 7.0 update and based on previous pattern, one can expect the update by November end or early December.