ONE's stories are primarily constituted by ninjas, psychics, demons, monsters crossed with household object, giant animals, superheroes with absolutely absurd abilities. It all smacks of something a deranged teenager scribbles in his notebook and the art reflects that.
Yet the characters and villains are the stupidest things you'll ever find legitimately entertaining, the tone is mixed up enough to keep both the stories and the gags fresh, and the amount of depth put into to his worlds and scenarios as well as the skill behind his paneling and general composition are surprisingly impressive.
The art style looks silly, but in a way that often works with the gags, and when he actually puts the effort in he draws some pretty badass stuff, like the "God" image and stuff from the Garou arc. I love it. I like it about as much as Murata's art but I'm a bit weird.
This dude gets that your art doesn't have to be great for it to complement your story, and that your premises and concepts don't have to be "serious" for your story to be legitimately interesting. This applies to basically all his works, so I might as well call this a general review. All of these stories are about super powerful "normal" people.
OPM:
One person is powerful but has a "normal" personality. He appears ordinary, so no one cares despite his living an relatively extraordinary life.
Off the bat there is a focus on a theme of subverting expectations and the story just runs with it. Sometimes it's predictable, a la the One Punch. Other times it's a character or development being significantly different in nature and relevancy than you would expect. Punchlines and twists are both frequent and both fairly well executed. It starts out slower, though not as slow as the next mentioned, but becomes fairly rapid fire in comedy in plot as it progresses.
MnO:
Nearly everyone is powerful while having "normal" personalities. They appear extraordinary, but no one cares because they all live relatively ordinary lives.
This one takes a while to build up, because it starts out mostly mundane with a little weirdness, before it starts to jump wildly between the two and mash them together in more entertaining ways.
MP100:
One person is powerful but has a "normal" personality. He appears extraordinary, so everyone cares but he wants to live an ordinary life.
In the first few chapters that have been (and are currently being) translated, it's really starting to flesh out a cast of characters in a unique way, which usually happens later in ONE works. Promising. It's also his main work, so it updates regularly.