I have 2 orange trees, one very older, over 5 years, and the other one just produced its first batch of oranges this month. Both trees are far apart from each other, grow beautiful looking oranges in color and size. Hoping so much the new tree was going to be right. You cannot eat them because they are as sour as a lemon. Climate here in Las Vegas is sun all day and cold nights, but this is the time they produce. My grapefruit tree, on the other hand, the fruit is sweet like sugar. I cannot figure why the oranges are so sour.
There can be many reasons for sour tasting oranges.
Oranges need heat to help build sugars in the fruit.
Harvesting to soon can also be a reason, test some of the fruit first and if they are sour, try leaving them on the tree longer. It is very common for people to harvest before the oranges are ripe. Generally if you wait longer, they will develop and sweeten up.
Check the variety of tree you are growing. They could be a sour orange tree, Citrus Aurantium, these are grown for their strong flavors and can be used to make marmalades and jellies.
Failure to remove suckers at the base of the trees can result in root stock take over. The root stock could be from a sour orange or even a lemon tree.
Here are some link to help you.