They indeed can oxidize. If skis are not waxed regularly after skiing, the wax that was originally in the base will no longer be there. Putting skis away regularly after skiing without re-waxing them will cause the molecular makeup of the P-tex closest to the surface of the ski base to harden up and oxidize.
When waxing an oxidized base, the wax will not go very deeply into the pores, and once the skis hit the snow, the wax comes right back out of the base. In a short time the skis will again be slow. To recognize an oxidized base, look for whiter, sort of milky-looking areas on the ski base.
If you think you’ve got oxidation on your favorite boards, all is not lost. Stonegrinding skis will tear away that oxidized layer of P-tex and open up a whole new surface thirsting to be waxed. Of course you've got a limited amount of potential stonegrinds in any skis, so its best to minimize this.