A Milchig food was put into a Pareve oven for a few minutes (on 350 degrees Fahrenheit)…
Later in the day cookies were put in, after the oven was previously heated to 350 degrees Fahrenheit, for about 5 minutes. Are the cookies allowed to be served in a Fleishig meal?
The cookies may be eaten at the meal – even if Fleishigs was eaten beforehand. They shouldn’t be eaten or mixed together with anything Fleishigs.
Regarding the oven, if you want it to continue being Pareve for the future, see this link for details on how to Kasher it.