To get to
Oaxaca from San Miguel by bus, first you'll need to take a bus to Mexico City. Primera Plus offers both first and second-class service (though you'll want to pay the extra for first class as the stops can become cumbersome) throughout the day. Check the
Primera Plus for schedules. From Mexico City, buses leave all day long until 11:30 p.m. to Oaxaca. Again, the first class bus is the ideal option as it takes the faster, toll highways. The journey from Mexico takes about 6 hours.
Click here for some more information
Transportation in Mexico.
I hope this helps.
Take care.