OnePlus One Omni ROM is the best android marshmallow ROM and comes with massive array of features. It is very stable but does not provides root by default. So you must flash super SU in OnePlus One custom recovery to get root. Here are features of it.
New boot animation
New calendar app added
DSPManager is back
DSPManager dark theme support
DeskClock dark theme support
Remove deprecated Settings options
Updated Launcher3 dark theme
Configure vibrate in camera launch gesture (experimental)
Updates to charger driver
Show carrier label and customize carrier label
Weather in Quick settings(Header view)
To install it, download the ROM onto storage > go into TWRP recovery and browse for the ROM’s zip file and flash it from there. Also flash gapps later using same method.
Omni ROM for OnePlus One is indeed one of the best firmware out there. It is bugless and works flawlessly on this android handset. If you come across any bug, feel free to chime in the comments section below.