My housekeeper mistakenly warmed up pizza (uncovered in an aluminum tin) for herself in my pareve oven. What is status of oven now? Can I revert it back to being pareve through running self clean for instance? Can I still use it for pareve food that I eat together with fleishigs?
Yes, you can use the self-clean function and this is sufficient for the over to revert back to a full parev status.
Even without doing this, there is room for leniency, since the pizza did not come into contact with the oven, and because it is a dry food.
But certainly after the self-clean function you can be lenient and treat the oven as fully parve.
See Iggros Moshe YD 1:40 (and 1:59, concerning baking a challah); see also Pischei Teshuvah, Yoreh De’ah 92:6.