Although Copernicus supposes these oscillations to take place around the orbits' lines of nodes that he assumes to remain fixed, the mechanism he uses to model them does cause tiny oscillations in the  lines of nodes as well. As Kepler later pointed out, the necessity for assuming oscillations in the inclinations of the outer planets' orbital planes is an artefact of Copernicus's having taken them as passing through the centre of the Earth's orbit. If he had taken them as passing through the Sun, he would not have needed to introduce these oscillations.
Although Copernicus believes these oscillations occur along the orbits' lines of nodes he assumes to remain constant, the model he employs causes slight oscillations in those lines as well. As Kepler later noted, assuming oscillations in the inclinations of outer planets' orbital planes is a result of Copernicus's treating them as passing through the Earth's orbit centre. If he had considered them passing through the Sun instead, he would not have needed to include these oscillations.