If you're already using extracts, that's probably the way to go. These will contain the aromatic aldehydes that give oranges their flavor/aroma and will mix well with water/syrup as they are in an alcohol solvent.
If you want to use essential oils, you will need something like guar gum to get the oil to mix with water. This basically contains the same aromatic compounds as the extract, but they're contained in the oil that actually comes from the orange rather than in an alcohol carrier.
You could use the orange juice, but this wall actually add less orange flavor, and more tart/juiciness (which comes from citric acid that is not contained in either the extract or the oil).
You could also try orange zest, which is basically using the peel of an actual orange and pulling the essential oil out of that, but you risk getting some of the bitterness from the pith(white part of the peel) along with your zest. To do it this way, just put the orange zest into your boiling water and strain it out before adding sugar.
Bottom line is any of these four methods will work, it just depends on the subtleties of the orange flavor that you want to accentuate in your finished beverage.
I hope that helps.