The Bed and Breakfast at the Learning Center is a great find. I have rented the apartment twice, and several of my friends have stayed in the guest rooms downstairs. The place is well located in the Historic Center, the guest rooms are immaculate, and the atmosphere is friendly and secure. The rooms are smallish but newly renovated, each has a private bath, and the price is more than reasonable. I would recommend The Learning Center to single women travelers particularly, as you have privacy, comfort and safety in a friendly atmosphere.
Gary Titus is a lovely and experienced host, and his cook, Andreas, makes wonderful breakfasts for the guests. Try his homemade granola and homemade yogurt with tropical fruit.
Staying here is just a little different from the usual, a window into Oaxaca you would not get in a hotel. The best part is your travel dollars go right into supporting a local non-profit organization.