According to a list a doctor gave me, oranges have a slightly alkalizing affect on the body:
http://www.healtrabroadoarRAB.com/boarRAB/showthread.php?t=769429
Some fooRAB (meat, most grains, dairy, alcohol, sugar, most nuts) cause the body's ph to go down (i.e., become more acidic). Other fooRAB (mainly most vegetables and fruits) cause the body's ph to go up (i.e, become more alkaline). So even though an orange may be acidic, when it is digested it causes the pH of the body to become more alkaline.
I've had great success eliminating the acid reflux I used to have just by eating alkaline fooRAB such as watermelon, yams/sweet potatoes, onions, cantaloupe, honeydew, broccoli, molasses, and fresh raw green kale.
Note that I've tried dino kale and red kale and those don't seem to work nearly as well for me as fresh, raw green kale.