The backstory of the Oxygen OS is rather long and outside of this topic. The original OnePlus One came running on CyanogenMod but in the end OnePlus had to come up with an original launcher and drop the Cyanogen entirely. This is how the Oxygen OS was born and it is now in version 2.0.
The Oxygen launcher aims to bring a nearly vanilla Android experience enhanced only by few, but useful software tricks. Since Android 6.0 Marshmallow is yet to premiere this fall, OnePlus had to implement a few features on their own before the official support is live.
OnePlus added fingerprint security support as well as drivers for the new Type-C USB port - both will be coming as part of Android 6.0. There are also configurable capacitive keys, a few nice gestures and some advanced UI customizations. Another Android Marshmallow feature available out of the box with the OnePlus 2 is the app permissions manager.
Most of the Android interface is intact - it's the Material design and icons we first met in Android Lollipop. All vanilla Android fans will be pleased to find out OnePlus touched almost nothing as far as homescreens and menu visuals are concerned.
OnePlus added four very useful gestures, which are available on a turned off screen. You can double tap to wake it up, draw O to open the camera, draw V to toggle the flashlight on/off, or draw < > for previous/next track.
Fingerprint unlock is available and it doesn't require to wake up the phone, just put your finger and you will be on the homescreen in a flash. Nice!
As of now, the fingerprint reader can only be used to unlock the phone, although that will change when Android Marshmallow is released and brings API for third party applications to use it in their apps, like on iOS.
The lockscreen is the usual affair - it shows missed notifications. Swiping left or right will fire up the dialer or the camera.
The Oxygen launcher has a feature called Shelf, which takes the form of your leftmost homescreen. Here you can see the weather, your most used apps, and frequent contacts. You can also add widgets here and change the header image. When you first setup the phone it asks if you want to use Shelf. You can also disable or enable it later through the homescreen settings menu.
The notification area is Android Lollipop by the book summoned by a downwards swipe anywhere on the homescreen. An additional swipe or a two fingers swipe reveals quick toggles that include Wi-Fi, Bluetooth, Airplane mode, screen auto-rotate, flashlight, location on/off, and the option to cast the screen. Of course, you can also adjust the brightness of the screen and go into the settings menu too.
The notifications also have one nice feature from Cyanogen, where screenshot notifications have a Delete button next to the Share button so you can instantly delete them from the notifications itself once you're done sharing them.
The app switcher has a neat card interface that allows you to select the app you need by swiping up or down. You can close apps by swiping left or right, or by hitting the dedicated button on the top right corner of each card. Each Chrome tab now can optionally appear as a card, too (Chrome settings). OnePlus added an option to close all opened/suspended apps at once.
Finally, Google Now is summoned by tap and hold on the Home key.
OnePlus added a few UI tweaks of its own - you can reverse the Menu and App Switcher keys, plus you can assign them custom actions upon tap&hold. The notification LED colors are also configurable. Each button has a list of things you can do for various actions.
OnePlus One also included a native support for app permission control. You can go into the Settings menu, check each app permissions, when they were used and if you want, you can disable some of them.
The final extra courtesy of OnePlus is the option to skin the entire Android OS with a dark theme and choose its accent color by your liking.