Orvieto in December

Short answer, yes, for a general December. But Dec 26 is a holiday so if you are specifically looking at tomorrow, there may be some closures. It may be chilly, but all over Italy, churches, museums, and other sites are accessible and uncrowded. We are in Rome now, have been in Italy all month, and find crowds are light and the weather good: onlt two days of significant rain in 3+ weeks to date!

Teal, I suspect Orvieto is going to be very quiet in December. St. Patrick's Well may be open, but it will be "chilly" at the bottom. The Underground tours will probably not be operating. The incredible Duomo is definitely worth touring. Don't miss Signorelli's Frescoes on the ceiling of the room at the end. Happy travels!