Orange Extract and Oil of Orange are staples in many recipes. Luckily these are pretty much zero carb, and have a lot of flavor per drop.
Oil of Orange is the concentrated form. It is made from orange peel, so it has all the flavor without carbs. You usually only use a drop or two of Oil of Orange in any case, as the flavor is so incredibly concentrated.
Orange Extract is a diluted form of this, with alcohol mixed in to provide a base. Again, it's zero carb - but because of the alcohol, you can't really use this for high heat cooking. It would just evaporate away the extract.
Some people even make their own orange extract by putting a lot of shaved orange peel into a glass jar with vodka and letting it sit for a few weeks. Then you strain out the peel and put the liquid into a dark container in a cool place.
Lisa Shea's Library of Low Carb Books