Is it safe to heat up the oven during these times or will it crack and crumble?
Jon in Montana
Did you make the oven yourself or is it a specific brand?
If you bought the oven, I would refer you to the manufacturer. Some ovens like the Portuguese ovens are no guaranteed to be frost resistant, I guess weather is always nice in Portugal
In most cases the oven should be fine as long as no water gets into the oven through the door nor the dome.
If is way cold outside, I would recommend making a small fire (flames that don't touch the dome) for a 1/2 hours or so the day before you use it. Then the day of you can fire it up all the way.
If you haven't used the oven in a while you should make a small currying fire for a couple of hours the day before using it.