Public Toilets — Oaxaca City, Oaxaca
Most places that cater to tourists will allow you to use their toilets, even if you're not spending money there. That said, in a pinch, underneath the gazebo in the center of the Zócalo, there's a subterranean public toilet, costing only a couple of pesos. (Z)
Toilets in México are notorious for getting clogged easily, toilet paper being the primary culprit. So next to toilets there are often waste baskets for disposal of toilet paper. Make sure to use them.