How is it possible to cook meat and milk in the same oven? Most of the observant people I know do not have two ovens, and they bake meat and chicken, as well as dairy foods, in the same oven (not at the same time, though). How is this done without making the oven "Treif"?

The Torah prohibits cooking meat and milk together. If you cook meat in an oven and then milk (or vice versa), you enter a problem, since sweat that comes off the meat sticks to the oven surface and then can mix with the milk dish. To avoid this issue you should try to always cover either the milk or the meat in the oven, similarly you should wait 24 hours since last used for one before you use the oven for the other, this will make the taste of the sweat non-fresh (Taam Lifgam) and reduces most of the problem. You should also heat the oven before changed from meat to milk or vice versa, to its maximum temperature for some 30 minutes to burn any substance that may have stuck to the oven surface and by that Kosher the oven for further usage. (Yabia Omer 5 YD 7, Igrot Moshe YD 3 10).