Android - How to flash Android O image in Moto G4 Plus
I'll supplement the answer of @beeshyams: since Android O is still in Dev Preview phase, the source code is still incomplete, and no custom ROM (LineageOS, etc.) could even base their work off it. LineageOS (back in their CyanogenMod days) also stated time and again that they will not touch Dev Preview code.
To reinforce the fact: until Android O is officially released (name confirmed and source code out), there's no way you can taste it on an unsupported device.
P.S. Historically, there had been ports of Android L and M Dev Previews to other devices, based on the image released alone. However, 1) such ports are often conducted between devices of similar SoCs, 2) those ports are nowhere near stable and usable, and 3) they quickly become obsolete once the proper release source code arrives.
If Moto themselves did not update your device, there is no way you can obtain an authentic system image. Even when your device is updated, Motorola is very slow to update the latest image on it's website, and you would have to depend on images obtained through unofficial means hosted on sites like XDA
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