Fans do some cool things. This is exactly what it says on the label. Someone via what looks like a YouTube comment asked these guys to make an anime OP for OPM, and they just busted out an original song just for that. I assume the song is called “Onepunch-Man Forever.” The whole video uses camera effects on manga pages and maybe even some .gifs you’ve seen before, but it looks pretty cool. Probably because Murata’s art and sense of motion could carry an anime OP on their own, but putting them all together looks nice. There’s also a few original drawing here and there, and the song being made in this style while being OPM’s unique thing is just too cool.
I found this via ONE’s twitter, so go like it a billion or so times so one day this might actually happen.