Can I update my OnePlus One (which was rooted to OxygenOS from CyanogenOS) to the latest AndroidOS?

[Russell Edwards1]

It will not update beyond Android 5.0.2

 

[Rukbat]

Because OnePlus never wrote any version of Android for it past 5.0.2. The manufacturer has to add hardware compatibility for stock Android to run on any given phone. There's a LineageOS for the phone, but it's still in nightlies, no stable version yet, so make sure you can put Oxygen back on it if there are too many bugs in Lineage for you. (And keep your eye on that page - they may finally finish a version.)