This exchange tells you everything you have to know about bus service in Oaxaca. It also tells you about when to cross the street, how to shop for a serape, and why everyone is so agreeable. Buses in Oaxaca leave from the center of town and travel outward. When they get where they are going, they sit while the driver chats with the inspector at the end of the line, and then they resume their roundelay. Occasionally, two drivers pull up alongside each other and chat for a moment, either verbally or by hand signals.
Schedules are far too restrictive, too un-natural.
You go when you can, stop when you must, and collect the fare from everyone who can pay it. That's not to say that there are no rules. If the bus is filling up, or if you don't feel like stopping, or if you don't like their looks, or if they are just gringo tourists who should probably be taking a cab anyway, you can ignore rule number one. This rule applies even to people waiting in bus stops. Every time you pass another bus, you will get his passengers.