It’s been few months since the release of Android 8.1 Oreo based custom ROMs, but a fully compatible Gapps package was not available until now. Without it you will not get full access to the Play store and other Google apps and services. Now Android 8.1 Oreo compatible Open Gapps package is available for download in both ARM and ARM 64 architecture, beta though.
For your information, Google apps such as the Play store are licensed by Google and are not permitted to be distributed as part of custom ROMs like LineageOS. However, you can download a GApps package and install it from within TWRP recovery, for Google functionality.
As you all know, after flashing a custom ROM like LineageOS, you will also need to flash Gapps package (the default Google apps) to get full access to the Play store and other Google apps. The Open GApps website is the best source for GApps package for your custom ROMs.
Before downloading the Gapps package, you need to select your device’s architecture (ARM, ARM64, x86, x86_64), Android OS version and Gapps variant (full, mini, micro, nano, pico). A free Play store download, ‘Open GApps’ app will help you in that.
The app will really help custom ROM fans to flash correct Gapps package (Google apps) on their device. Interested users go to Play store here and download the app. You can download Open Gapps from here.