Output Impedance of a DIP8 Crystal Oscillator
Assuming it's a digital output, not sine wave output, it's typically not specified in the data because it's not terribly useful and it isn't constant- it will be different for high and low states, for example. It will also vary with supply voltage etc. It will also be lower for light loads than for heavy loads.
Probably somewhere in the 100 ohm range, give or take 75 ohms. With a light load a CMOS buffer is more like 15 ohms, but working into a low impedance it's more like a constant current source.