OnePlus recently announced their intentions on building their own ROM as a replacement for the Cyanogen OS for their mobiles. And as a follow up they have now released an alpha build of the ROM based on Android 5.0 Lollipop for the OnePlus One. The ROM is available on their forums right now for download and flashing.
The ROM is as close as possible to stock Android Lollipop on Nexus devices with few minor changes. Since this is an alpha build, there are quite few bugs listed on the forum. Head over to the OPO forum for clear instructions on installing the ROM and the pre-requisites for the same.