Any idea what to do with a whole bunch of ornamental (sour) oranges? They're very sour, about the size of a golf ball or smaller.

Another vote for Yucatan Pork. (I use a pork hock...hind leg if possible). You will need some Achiote - usually a small brick of compressed powdering spices and peppers. Find in a mexican deli. You toss that into a blender with the juice of the sour oranges. (Google a recipe). And, wrap in banana leaves and bake in the oven. Banana leaves are usually in the frozen section of an asian deli. I did my yucatan pork for 5 hours at 325F and the meat was falling off the bone delicious.

If they're similar to Seville oranges, then this Nigella recipe is DIVINE, and very easy!
http://www.nigella.com/recipes/view/no-churn-bitter-orange-ice-cream-176

If they are anything like the bitter oranges that are all over Athens, Greece, you could make a beautiful preserve from the skin. (The trees are everywhere in the suburbs and the fruit goes unused for the most part). Google bitter orange "spoon sweets" for recipes. One local recipe strings the curled up slices of skin on a thread before poaching them. Quite lovely when finished. A friend also made a nice marinade with the juice for pork tenderloin w/ olive oil and capers. Another recipe from Lesvos has you grill pork chops with halved bitter oranges on the grill (cut side down). Then squeeze the juice from the grilled and slightly charred orange half over the chop while it rests.

They might be calamondins. They're used in Asian cooking but you could candy them or poach them in a simple syrup.

My dear late grandmother had such a tree, and used them in cocktails instead of limes. I always thought her to be hopelessly sophisticated and charming- if cocktails aren't your thing, maybe in fizzy water or an orangeade?

2nd vote for Cochinita Pibil. Aren't you lucky to have the oranges already on hand? Or if you have a lot, I'd make some marmalade.

Maybe try preserving them with salt - like Middle Eastern preserved lemons - to use in tagines, etc.?

Achiote chicken!

Cochinita Pibil! Much Yucatecan cooking calls for sour oranges (as does Cuban) I would recommend searching online for some things to cook from those regions. My recipe combines orange and lime juice but your sour oranges would work great:

https://food52.com/recipes/3377-cochinita-pibil

Since the oranges are so sour, I would make mojo and marinate some meat in it.
http://icuban.com/food/mojo.html