La Cumbre Ixtepeji is an ecotourism center located NE of Oaxaca and accessible via the highway 175. It is the largest and best developed area for downhill mountain biking in Oaxaca and features ~9 singletrack trails.
La Cumbre has an entrance fee of 50 pesos per person which can be paid at their oficina here. From there, you can contract a guide to drive your car for the day (~200 pesos per person) or pedal up 👊.
La Cumbre Ixtepeji can be accessed via public transit from either the second class bus station or alongside the road near Monumento Benito Juarez. There will be busses and collectivo taxis regularly making the trip from there up to La Cumbre and Ixtlan. Just make sure to let the driver know where you need to get off. Mountain bikes can sometimes be strapped onto the roof but this will depend on the driver and how full the bus is.