Buck converter with op-amp, inductor issues when load changes
For any given load, a switcher will transfer a given amount of energy thousands of times per second. This is how the buck regulator works.

Let's say your op-amp is switching at 10kHz (because it's a slow sort of device and will have slew rate problems compared to other devices). Let's also say you are aiming to deliver 5V across a 10 ohm resistor. Resistor power is 25/10 watts = 2.5 watts. 

To calculate energy per switching cycle divide this power by frequency because power = joules per second. At 10kHz, the energy you transfer per switch cycle is 250\$\mu J\$. 

This energy powers your load resistor but, if you removed your load resistor, this energy gets dumped into the output capacitor and its voltage rises a little (or a lot) higher than normal.

Let's say your output capacitor is 10uF - if suddenly it was imbibed with 250\$\mu J\$, how much would it rise in voltage?

We know that capacitor energy is \$\dfrac{C V^2}{2}\$ therefore we can calculate the voltage rise and this is: - 

\$\sqrt{\dfrac{250\times 10^{-6} \times 2}{10\times 10^{-6}}}\$ = 7.07V.

It's a little bit subtler than this - in the above I assumed the capacitor was being charged with energy from a zero voltage state. In fact it already has 5V across it and this means that the previously stored energy + influx energy (from the inductor) is 125\$\mu J\$ + 250\$\mu J\$ = 375\$\mu J\$. 

If you do the reverse math, the peak voltage on the capacitor becomes 8.66V i.e. 3.66 volts higher than the 5V rail. 

You could put an argument together to consider the losses in the diode also - this may trim half a volt of the absolute peak voltage.

So, you either need to increase the capacitance a lot or, decrease the transfer energy by increasing the operating frequency. Modern switchers regularly operate at 500kHz and this means the energy per cycle reduces from 250\$\mu J\$ to 5\$\mu J\$ in this example.

Should this be the case (500kHz operation), the rogue energy from the inductor would make the capacitor's stored energy 130\$\mu J\$ and this means a peak voltage of 5.1 volts - probably quite acceptable for load dumping on a switcher.

Operating at higher frequencies requires faster silicon but, the ability to control load variations (and their repercussions), on a cyclic basis, means much tighter control of the output voltage.

This is just an example to see where you might be going wrong.