I clicked Tony's link above to see where it led. The info says it is 18km to city center. We stayed at Oaxaca Campground last winter for several weeks and drove in and out of town many times. It sure did not seem to be as far as 18km. It is a very easy in and out of town - as well as to other local attractions. But having a car would be desirable. I think there is a bus stop near the campground and there are taxis as well as tuk-tuks. The road in and out of the campground is a little rough but easily managed at slow speed - a short distance.
You can see Oaxaca Campground on Google Earth at 17.030503 -96.599031 . In fact I think that is our Bus in the picture.
Here is a Google Earth image of the pink paint store at the intersection where the turn is made to the campground from the highway.
Click For Full-Size Image.