Oculus has decided to open source its Rift Development Kit 2 ahead for theirs Oculus Connect 4. This is done to promote the developers to make their own add to it with their own designs. The second generation of development kit was released in the mid-2014. Development Kit 2014 was a massive improvement over its predecessor with a 1080p OLED display and the positional tracking system (via external camera). This also laid the foundation for the first customer version of the Rift which was later released in 2016.
Oculus has released all the relevant documents on their official website. The documents include schematics, layouts, mechanical CAD, artwork and specifications under a creative commons Attribution 4.0 licenses and firmware under PATENT + BSD licenses. These things are very much all that one could need to make their own version of DK2 outside of a 3D printer. But one of the limitation is that it might be very difficult to get your hands on some of these components anymore.
Even the company excepts that it would be challenging to build the complete headset from the provided files as the required components may no longer be available. The company hopes that this release would be useful for the learning purposes if nothing else.
The current version of Rift is also a big improvement upon DK2 with a much higher resolution, better tracking, and Oculus controllers. So, it might be few years that the company also decides to open source the current version. But we will have to wait longer for that to happen.
Rift DK 2 release blog: developer.oculus.com