Fairphone OS is just the name of Fairphone’s own Android OS on their phones. Fairphone OS is Android, at different versions on the different phones.
Fairphone OS on the Fairphone 1 was Android 4.2 when it ended officially.
An early Android 4.4.2 testing version exists, which couldn’t be finalised because of missing support from chip manufacturer MediaTek … FP1: Macadamia 1.9.9-pre3 Alpha (Android Kitkat).
And an unofficial community effort of Android 4.4.4 exists … Android KitKat (4.4.4) for FP1.
Attempts at a usable Android 5 on the Fairphone 1 went nowhere.
So that’s it. If an App starts needing Android 5 minimum, then you can not run it anymore on the Fairphone 1.
You can not install the Fairphone OS for the Fairphone 2 or 3/3+ on the Fairphone 1, this is technically not possible, it doesn’t work that way.