Use of Capacitor in output of op amp
C1 is a coupling cap, C2 serves to provide the high frequency charging current required by the ADC sample phase. 

Usually ADCs are a switched capacitor architecture and draw large current pulses from the input during the sample phase, you want the source to be low impedance at radio frequency to accommodate this, hence the small ceramic right across the pins.

You may want to put a few tens of ohms of resistance in series with C1 to isolate the opamp from the load imposed by the 100nF cap for stability reasons (I would make C2 more like 2.2n or so, 100n might be causing a lot of HF rolloff). 

I would note that it is a horrible circuit for other reasons.