OnePlus announced its first smartphone back in 2014 and since then, the company announced four new smartphones. Considering OnePlus One is a fairly old smartphone, OnePlus has no plans to release Google's latest mobile operating system for the smartphone.
The good news is that there are people working on the update. XDA senior member Agent_fabulous has managed to take the Oreo AOSP and create an unofficial and experimental build for the smartphone. It is still in early development stage, so expect to experience some bugs, but that will be fixed with new releases.