How do I reheat a casserole in the oven?
There is no single answer.  It depends mostly on thick the casserole is, and how cold it is.

For a not-frozen casserole, I recommend starting with about 3/4 of the original bake time at about 350 F, and then seeing if it is hot enough.  An instant read thermometer in the center should read about 150 F to 160 F for service.

You may need to put foil over the top to prevent over-browning, or increase the heat at the end to about 400 F to encourage more browning, depending on the nature of the particular casserole.

Since you are just heating it through, generally any temperature from about 300 to 400 F is going to work, with an appropriate adjustment to the time, if there are other items in the oven that need a specific temperature.