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.
At the moment, OnePlus One runs on Oxygen OS which is a modified version of Android Lollipop. There is no hint of OnePlus One getting Marshmallow. But OmniROM team have created a custom ROM running on Android 6.0 for OnePlus One. If you want to install it, follow the guide below.