OnePlus launched the OnePlus 2 last year with Lollipop, and there it has stayed all through the launch of Android 6.0 Marshmallow. Some of the more established OEMs have already gotten 6.0 updates out the door (even those that have to go through carriers). Now it's almost OP's turn to update devices to Marshmallow. There's a beta for OxygenOS 3.0, which is based on Marshmallow. You can install it right now, if you like.

OnePlus calls this a "community build" because it wants members of the community to install and test it. It won't be sent out OTA, so you'll have to download and flash the installer. Here's the changelog posted by OP.

  • Android 6.0.1 Marshmallow upgrade
  • New OnePlus Camera UI
    • UI has been cleaned up: swiping up/down now switches between photo/video, whereas HD/HDR and flash settings live in the top right corner
    • You can now change the aspect ratio and toggle grid view without going all the way to the settings
  • New wallpaper picker
    • Easily preview and set both home screen and lock screen wallpaper in one place.
  • Shelf UX has been improved
    • No more edit mode, long press boards to rearrange or remove
  • General system performance improvements
    • Benchmarked 47% faster than OxygenOS 2.2.1 on key test scenarios
  • Fingerprint performance improvement
    • Tested 45% faster than OxygenOS 2.2.1 on average
  • Settings changes
    • Old app permissions are gone, say hi to new (native Android) app permissions.
    • Alert Slider has its own entry point with more customization options
    • Improved settings menu readability by changing the order a bit
  • March security patches

You can install this 1.1GB build on the OnePlus 2 as long as you are completely stock and running v2.2.0 or higher. Flashing back to the Lollipop build of OxygenOS will require a complete device reset, so make sure you back things up. OnePlus also cites a number of potential issues.

  • Your fingerprints will not be preserved after you flash the new build, so you will need to re-register your fingerprints if you have fingerprint authentication set up
  • Maxxaudio tuner and presets will be removed
  • Rolling back to previous OxygenOS versions will not work without wiping data
  • Some soak testers have experienced wifi speed issues which we are investigating
  • Some of your old apps won’t work without an upgrade to M compatible versions
  • Shelf frequent apps board size might shrink if you are dirty flashing. Remove and re-add the board and it will fix itself
  • Language localization is still being improved

All of the changes with fingerprints are due to the move from OP's in-house solution to the standard Android API. The OnePlus permission manager is also gone, replaced with Marshmallow's native permission settings. It's not clear how you'll go about updating to the final build when that is available. I'd bet on a full system image flash, so you might want to hold off if you don't want to deal with resetting your device. Scratch that, OP says you'll still get OTAs.