gonebabygone: Two things.

1. I've taken to searching ITA while explicitly turning off the option "Only show flights and prices with available seats". This way, it's easy to verify that it is in fact theoretically possible to follow the fare basis code you mentioned on a trip that went from, say, May 21 to May 28. So that's a good start.

2. After seeing that that's possible, you then have to check availability. By turning back on the option "Only show flights and prices with available seats" and performing the search again, we notice that flights are no longer found. By taking each flight segment individually and checking availability we see that there is O-class availability for the YMQ-LHR flight on the 21st but no O-class availability for the LHR-LYS flight on the 22nd. So that's what's stopping you from booking at this fare, I'd say.