Power supply selection circuit review
I'd suggest you are way overthinking the need for isolating the power sources. 

As a note: Your linear regulator seems to be a 7818DT ...that is an 18 Volt regulator. Did you actually mean to use a 7805 regulator?


Assuming that the linear regulator is in fact a 5 Volt 78xx version regulator, you need to provide backpower protection. Put a diode between the Fuse and the input capacitor to the regulator. Anode to Fuse, Cathode to cap/regulator. 
Assuming your USB power input is a USB output Walwart you don't need Q1 to isolate it. Just as you don't have a diode in the output of the 5 volt regulator, it makes no sense in the USB line. If the 5 V provided by one regulator is above that of the other, the respective (other) regulator will simply shut down.
If the USB power connection is from a computer/tablet/etc then they will either current share, or the upstream provider will shutdown as per standard.
Either way you achieve nothing but increased complexity by adding Q1 and the op-amp. 


If you really really really  want to have isolation, then simply add a Schottky diode in series with the USB power input.