OnePlus tried to assuage fears of a stagnating OS experience a few months ago during the CyanogenOS fallout by announcing it was working on a ROM for the OnePlus One. Eventually dubbed OxygenOS, the company has been working on getting the software out the door for several months. OnePlus promised to have it done by March 27th (today), but that's not happening. Bummer.
According to OnePlus, the delay is due to additional quality control testing. The ROM supposedly works just fine, and multiple people are running it internally without issue. It will be released once OnePlus is confident it will pass Google certification and all OTAs will go smoothly.
To make up for the delay, OnePlus gave away five phones, which it previously pledged to do if it missed the deadline. Those winners are listed in the announcement post. For everyone else, there will be daily progress reports posted on the OnePlus forums. OnePlus' head of mobile product says no bit of news is too small to escape mention. I guess we'll see how that works out.