Can a Chihuahua eat oranges?
Better to skip: the acidity doesn't agree with him.
How much: If you really want to, 1-2 segments without peel or seeds, very occasionally.
Why
Oranges aren't toxic, but citric acid upsets the stomach of many Chihuahuas. The peel is harmful and should be avoided.
Oranges in a Chihuahua's diet
Keep in mind how tiny the portions are for this breed: an adult Chihuahua eats roughly 1 to 2.3 ounces (25-65 g) of food a day depending on his weight, and treats shouldn't go over 10% of his daily calories. That's only a small fraction of an ounce. What looks like "a little bite" to us can be half a meal for him.
This information is for general guidance and is not a substitute for advice from a licensed veterinarian. Every Chihuahua can react differently; if in doubt, always check with a professional.