My vote is for squirrels. Between them and my my deer, it's amazing I even have a garden. Preventative measures (non-violent, I assume)?...offer them dried corn on the cob and they'll eat their fill there. Then they may leave your oranges alone. Of course, you can always cart the tree inside every night...what a pain! =)

Deer would eat the leaves, a raccoon would knock over the pot, opossums aren't interested, so, probably squirrels.

squirrels are very destructive to immature and mature fruit, in my experience. Another vote for them. As for what you can do- try spraying/sprinkling the bush with hot pepper. You can also cover it with netting- but keep the netting off the bush or they'll gnaw through the net to get to the fruit (i found out the hard way last year). I have read that leaving some exlax out for them in bits in the pot will deter them- they eat the exlax, get sick and associate the pot with getting sick. They have eaten the exlax for me, but it didn't really deter them from coming after the fruit later, so i'd take that one with a grain of salt. Best of luck!

I must have jinxed myself by replying to this post, because yesterday morning I saw a squirrel on my potted lime tree, ripping off an immature fruit. Squirrels are playing havoc with my garden this year...they're almost up there with the deer, they're so bad!