During extremely cold weather, wildlife need to eat more to stay alive. The opossum likely is not finding enough at night and is out looking for food during the day. That isn't uncommon. I'd be more concerned for the opossum than the dogs if they did run into each other. An opossum will "hiss" and show off its teeth when threatened, but that's pretty much all they have got. If that doesn't scare the predator/dog off, it will then "play dead" and hope the predator leaves it alone. As you can imagine, with a dog, that's not a great defense mechanism...more like a good way to become a "chew toy" if the dog is large.
