First off - it sounds like its a calamondin (citrus mitis) which is not really an orange - they are sometimes sold though as "miniature oranges". The fruit will be smaller (maybe a little smaller than a golf ball), and they will be much more sour.

As for the care: Does it have any bugs on it? Anything sticky? If not and the leaves are yellow check 2 things:
1) Make sure the soil is draining very well. If its getting a lot of moisure outdoors this is even more essential. Consider pulling it out of the pot to see if the roots are healthy, and consider re-potting in a more free-draining mix if required.

2) Have you fertilized it lately? If the leaves are yellowing it could be a nutrient deficiency. With citrus there are a few things to note. First, citrus do require micronutrients - so feed at least occasionally with a fertilizer that has these - a fruit tree / citrus fertilizer should contain these. Second, citrus need acidic soil in order to efficiently take up nutrients. A good way to maintain that pH is to use a fertilizer for acid loving plants like camelia / rhododendron fertilizer. The common one that I use (and many others) is the MiracleGro sometimes sold as "Miracid" and sometimes sold just as camelia / rhododendron fertilizer.

My $0.02. Others may have more info.

Calamondins are pretty hardy. A friend's mother has had one for 20 years and keeps hers in a dark garage all winter and it still does fine!

Good luck!

Pelham

I agree with Pelham on the soil mix. Citrus hate waterlogged roots. Quick Draining is key. You should try to get the little guy in full sun. I have 2 Calamondin's that are in 11 hours of sun and they are going crazy with new shoots. Your little guy will perk up!

Fruit will be sour, good for making drinks and some people cook with them.

My Wife and I's fav drink this year: Calamondin Cairpirhania - 4 Calamondin's Sliced, 1 Shot Lite Rum or Cachaca, 7-10 counts of simple syrup (Sugar water)

Muddle the Calamondins, add Rum, add simple syrup, add lots of crushed ice, pour into glass, enjoy summer.

Citrusboy aka Marc

I ate one of its little oranges and once and you want to talk about SOUR! I found it inedible. Maybe in drinks they'd taste better.

By the way, do you have to pick the fruit off the trees? Does leaving them on hurt the growth at all?

Baby, yes the fruit is awfully sour..ppl make jams and marmalade from these fruit. If you're into making jams there ya go..(S)
You can leave the fruit on the tree until they fall naturally or remove.
I bet your lips puckered when taking a bite of that orange..lol..Toni