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 One also comes with Oxygen OS in addition to modified Cyanogen OS. If you want other options, there are many custom ROMs. The official CM 12.1 ROM can be installed on OnePlus One. This is stock CM 12.1 and it varies from the modified Cyanogen OS present in OnePlus One. Follow the guide below.