Why does the voltage of my Buck converting SMPS drop when a load is added at the output?
The main problem is the 741 op-amp - you appear to be powering it from a 7.5 volt zener diode yet, if you read the 741 data sheet the implied minimum supply voltage is 10 volts. See table 7.2 in the uA741 data sheet. Try using a modern device and not this crappy dinosaur. Think "rail-to-rail".

Also, I don't see any decouplers on the op-amp power rail.