In most recipes you can switch old fashioned for the quick oats and vice-verse, but as others have said, stay away from instant. It has added ingredients that might throw your recipe out of whack. I have even used old fashioned oats in a bread machine recipe that called for quick oats, and it comes out just fine - maybe better. Another thing you can do if you have only the old fashioned oats around but want to use quick cooking oats is put them in the food processor. If you run it too long you get oat flour. Seriously, all the quick ones are is old fashioned ones that have been chopped up a bit.
Another thing about instant oatmeal is that the "real" thing - old fashioned rolled oats - will cook almost as fast in the microwave. Instructions are on the package. Use a big bowl because they boil over very easily.