0-volts (lean) to 1-volt (rich).
At 2000 RPM (engine at normal operating temperature), specification is 0-0.3v on the low side and 0.6-1.0v on the high side, with lean-to-rich cycling changing more than 5x in 10 secs. The oxygen sensor, itself, generates the voltage. The ECM has an internal fail-safe for the oxygen sensors; if it doesn't read a voltage from the sensor, it faults to an internal 0.33 volts for a reference.
This is a little different in operation. Technically, the answer would be minimum voltage of "less than 1-volt" and maximum voltage of up to 5v. The sensor is fed battery voltage and sends a signal voltage to the ECM. Specifications call for (with engine warmed up and running) 1.0-1.7v @ idle and 1.7-2.3v @ 2500 RPM. It would be less than 1.0v with key "on" engine "off" and approx. 4 volts @ 4000 RPM.