Stability problem in unity-gain opAmp
Your op amp is oscillating because your open-loop gain is larger than 1 at a frequency at which the phase shift is 180°. 

The op amp in your circuit is driving an almost entirely capacitive load - the MOSFET's gate. 

There are many possible ways to correct this using just a well-placed resistor or a capacitor. It might be best to use a series resistor or a parallel RC shunt, or a feedback RC pair - it all depends on the particular circuit in question.



For more on this, refer to this excellent article by Analog Devices.