It took a lot longer than expected, but OnePlus One owners finally got the official Android Lollipop treatment a few months ago. Now, the latest version of Android, which is 5.1, will hit some devices rather quick, whereas others may have to wait quite awhile.
Currently, CM12.1, the CyanogenMod OS for the OnePlus One updated to Android 5.1 is currently undergoing testing and Google certification. A solid release date is not given, but should be out before the end of June. Currently the team is running a preview build, but if you’d like to install it on your OnePlus One yourself, you can find the link here.
As for OnePlus’s in-house ROM, OxygenOS, things are not as promising. According to Helen on the OnePlus forums, the entire team has their “heads down working on the release of the OnePlus 2.” There will not be an official OTA for OxygenOS until the OnePlus 2 is released, but they will have a preview build up soon.
OxygenOS updates will come more promptly for the OnePlus 2 as it will be the official operating system. Once the next device is released, the OxygenOS team will be expanded to help expedite updates. They will also take more suggestions into account from the forums.
One thing to note is that because they are stalling the update until release of the OnePlus 2, an announcement must be on the horizon. They are probably shooting for a July release, as August would bring in Android M. Keep a sharp eye out.