I'm just going to get this out of the way first. OES is not okay fill, especially when clued as "Latin diphthongs". Maybe that's tongue in cheek. Maybe there just wasn't anything else that was going to work. Maybe he just had to swallow the fact that he had OES in his puzzle, and he came up with a clue that was so ridiculous, it's funny.
I say that I just want to get that out of the way, because I flat out loved just about all of the rest of this puzzle. Yes, there are a few other entries I'm less enamored of, but I can accept LAI, EPH, ONS (really, as a partial?), or PEI when the theme is so excellent, and some of the cluing is outstanding, including my nominee for best clue of the year (and maybe the decade), which I'll get to in a bit.
I struggled for a while with the puzzle. I had scattered answers, with very little in the way of momentum. It became clear why that was long after it should have. I mean, I filled in the entire SW corner, including PEPSQ/UAD. I stared at PEPSQ, and thought to myself, "Huh. That doesn't make sense. Maybe it'll become clear later on."
I even thought, "Shouldn't that be 'pep squad'?"
Well, they don't pay me to solve these puzzles.
Anyway, I got it when I filled in 61A: Prompt... or a hint to entering five answers in this puzzle (RIGHTONCUE). That's awesome. All of the theme answers are good phrases with a Q buried inside, and each of the right turn answers are answers in their own right, from QUAD to QUIPPED to QUASH to QUESTS to... yeah, I'll get there.
Some fine clues include 49A: Jersey delivery? (MOO) and 9D: Swift, in music (TAYLOR) which threw me for a long time, especially since it was right next to INE (8D: Having four sharps). Very nice hidden capital there. Not far removed also, from Taylor Swift's good friend, LORDE. 34D: Cause of some 911 calls (UFO) is a nice misdirect.
But the best of all comes at 10D: Army terror? (GIANTS/QUID). I'm still giggling at that one.
Thumbs way up.