That looks like expected behaviour to me (OxyPlot is designed to maximise use of space without clipping anything and doesn't cope well with artificial constraints: this can lead to weird and annoying behaviours if the data change). Looks like you have pan and zoom disabled?

In your situation, I would manually set the PlotMarkgins explicitly and reduce the IntervalLength (screen space dimensions) to ensure there is always a label on screen (or use the new MinimumMajorIntervalCount); this will however, lead to cropped and so potentially misleading labels on the right-hand edge if a generous enough margin is no chosen.

plotModel.PlotMargins = new OxyThickness(double.NaN, double.NaN, 50, double.NaN);
xAxis.IntervalLength = 50;

Certainly the behaviour could be better here. An option to force the pan/zoom behaviour (which leads to more stable, but not entirely predictable margins) might be useful.

For interactive plots, I usually slap a fixed and wide margin on them; there's no real alternative if you need total stability. A pair of MinimumPlotMargins and MaximumPlotMargins properties to clamp the axis margins would be useful and easy to implement.