Some say its ok, others will tell you citrus fruits are not great for dogs. Oranges are defiantly not toxic but they can be overly acidic for some dogs and cause diarrhoea so they should be fed in moderations, and without the peel of course.
Also see Vitamin C
Orange, lemon, grapefruit and lime rinds contain rutin, a plant pigment (flavonoid). It is believed rutin may have anti-inflammatory effects. Grating some well washed organic rind of grapefruit, orange, etc adds rutin to your dogs diet.