It's funny how you say "old" about a device that is with you for 5 years. Can you say the same about a child? :) It's all relative... But anyways, I was thinking about firmware upgrade for my Oneplus One for quite sometime now, but always felt insecure about it. Was worried it might be unstable or just the sheer fact of spending whole day importing everything from backup and setting all up the way I like it - these things have been throwing me off - call me lazy if you want :) Anyways, new year, new hopes, so I kind of took another look and found out LineageOS had really good opinions in the community and was known for its stability. For me it was also important NFC, GPS and Bluetooth work flawlessly as I heavily rely on them during my travels. I'm aware there are other interesting ports for my device like for instance POSP (Potato Open Source Project, here's a good name, right? :)) or Pixys OS, both based on latest android installment 9.0 Pie. But like I mentioned I wanted something more ... solid in terms of what's working and what's not, so thought I'd stick to 8.1 (Oreo) for now and see how that goes. And I have to say, it works pretty well. So far had only one issue with Google Photos app that kept crashing, but reinstall solved it, so no other complaints. Few days back I have also replaced the battery, am pretty impressed with how my new OS handles apps, perhaps it's very subtle but it seems as if my phone can run longer on a single charge (this is probably also due to the fact I have clean install with less bloat and far less apps - that I had not used anyways when on my old firmware).
Anyways, long story short: if you, like me are a happy Oneplus One user and thinking about a change do not look further, LineageOS is definitely a good, reasonable choice. Nightly builds can be found here:
Google Apps (I'd recommend micro package as a base):
(choose ARM, then 8.1 and then micro).