LineAnnotation on multy Axes

There are many elements of the Axis, we must somehow specify the element LineAnnotation what used to work the same way as is done in LineSeries:
<oxy:LineSeries ItemsSource="{Binding Items}" MarkerType="Circle" Color="DarkBlue" YAxisKey="Length" XAxisKey="Volume" />

How to do it for LineAnnotation?

You should use a Plotmodel from the OxyPlot Core library. Indeed some element in the WPF library are not finished yet (Annotation don't have axis key in this library, but they have in the core library).
So build your chart on a PlotModel in your code, then set the chart model property to this Plotmodel.

I hope this will help