OnePlus One sports a 5.5″ 1080×1920 JDI display. It’s powered by a 2.5GHz Qualcomm Snapdragon 801 processor and 3GB of RAM. Storage comes in at either 16GB or 64GB. Other features include a 13MP camera, 3100mAh battery, dual front speakers and StyleSwap replaceable back covers. On the software side, the OnePlus One runs a version of Cyanogenmod with a unique modified user interface.
OnePlus has not pushed out Android Marshmallow update for any device yet. However there are many custom ROMs like CM 13, PACman, OmniROM for OnePlus One. Another ROM added to the list is AOSP ROM running on Android 6.0. If you want to install this ROM, follow the guide below.