Prophets, harbingers, and doomsayers (and chosen ones) are their own sort of overused plot device; OPM ran a pretty standard "Fainting Seer" and "Vagueness is Coming", then partially deconstructed those tropes by having Shibaba die of choking and having the cataclysmic, but vague threat show up in clear form minutes after the revelation.
The simple existence of a prophecy probably doesn't count as a MacGuffin; there are way too many clear-as-daylight MacGuffin plots. Off the top of my head: The Tesseract. The Galaxy on Orion's Belt. The Holy Grail. The Ruby Slippers. Marcellus Wallace's suitcase. The Maltese Falcon. Casablanca's letters of transit. The meaning of "Rosebud." The portrait of Dorian Grey. Excalibur. The Golden Fleece. Elemental crystals, philosopher's stones. Magic rings and Shannara things, Sansa Stark and the Covenant Ark.
They don't have to be entirely useless; they don't have to be physical trinkets; prophecies can also be involved. But, the MacGuffin itself is an arbitrarily-inserted object that exists for no other reason than to give the characters a reason to go do things. If you could reasonably substitute many sorts of Things of Great Value for the thing in question without meaningfully changing the story, you're probably dealing with a MacGuffin.
The closest thing to a MacGuffin I can think of in OPM is probably the big sale that Saitama was after during the House of Evolution story. Saitama was frustrated with the intrusions of the chimeras, but he decided to go deal with them right then because he didn't want to miss the big sale. Of course, the deconstruction/joke is that for anyone but Saitama (and life-partner Genos), it'd be a ludicrous reason to hastily challenge a powerful criminal organization.