If observing from an Activity you can observe on onCreate() and use this for the LifecycleOwner as stated here:
If you have a lifecycle-aware component that is hooked up to the lifecycle of your activity it will receive the ON_CREATE event. The method annotated with @OnLifecycleEvent will be called so your lifecycle-aware component can perform any setup code it needs for the created state.
Now if you are observing within a Fragment you can observe on onViewCreated() or onActivityCreated() and you should use getViewLifecycleOwner() and here is why:
Get a LifecycleOwner that represents the Fragment’s View lifecycle. In most cases, this mirrors the lifecycle of the Fragment itself, but in cases of detached Fragments, the lifecycle of the Fragment can be considerably longer than the lifecycle of the View itself.