You cannot simply install an "Android 8 APK" like a normal app. Android is an operating system, not an app. However, there are legitimate methods using APK-based tools (custom updaters, ROM managers, and recovery installers) to upgrade your OS. This guide will show you how.
Upgrading from Android 7 (Nougat) to Android 8 (Oreo) cannot be done by simply downloading and installing an APK file. Android version updates involve deep system changes that require a full system image or an Over-the-Air (OTA) update package. Upgrade Android 7 To 8 Apk
You are not installing Oreo via an APK; you are installing a tool via an APK that facilitates the upgrade. You cannot simply install an "Android 8 APK"