What is resistor R2 doing?
Op Amps driving switched voltages with low impedance output and a capacitance load causes a loss in phase margin and oscillations.  Adding addition Miller capacitance with a series R outside that loop improves the phase margin at unity gain so the output has a clean square wave.

To demonstrate this I modelled the OPA2172 CMOS RRIO OP AMP with 60 ohms output is in the datasheet. Without the C1 it oscillates even with the series R and dummy load capacitance.

The Zener prevents the load capacitance ringing from back driving the CMOS output above the supply rail which could cause shoot-thru failure.



Adding 50 ohms in series to a load cap before negative feedback is the not fix, unless there is ALSO some > 1pF negative feedback from stray capacitance. 

This means if there was stray capacitance to the +ve input, it could still oscillate (IF IT did not have the shunt load cap on Vin+ which also LPF's the input)

Zout is the open loop output resistance given in the datasheet of 60 ohms.