I'm not sure, but I think your tree has somehow cross-pollinated. Are there any lemon trees around your orange tree?

Do you know what variety of orange tre this is ? If it's a Valencia , the fruit does not ripen until May so it may taste sour because it's way to early . Lots of orange varieties ripen much later than early December .
On the other hand , if a tree is sour naturally there's nothing you can do to change genetics . Just plant a new variety !

Cross pollination WON'T make the fruit sour.

You may have a sour orange that is used for root stock.

I had a tree like that, but instead of ripping it out, I grafted minneola tangelo to it - outstanding!

Here's a photo of the sweet lemon that I "top-worked" by grafting Minneola tangelo:

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It has tangelos after only 2 years of grafting.

Well, my great grandmother has a lemon tree and an orange tree in her backyard, but the lemons were sweet and the oranges were sour. Just a thought...