One i forgot is What Came to Mind During My Third Time in Another World Was to for Now, Get Naked - basically summoned as a hero then saved the world and got new game + and saved again and got another new game plus. (starts op) also at worst the rating for this in America would be pg-13 at least as far as I've read.
I want to ask what kind of OPness are you looking for, or like. Like Kumoko, she is "OP" but only after struggling for a very long time to turn from weak to a little OP to OP but still getting her butt almost kicked to OP. Now, she is top tier. Then there is the MC is OP but only to his generation, example is ATG. MC is strong for his age and generation. So to get his ass kicked, author throws in old geeks. Just look at almost any Chinese wuxia and xianxia. All of those are similar in that MC is OP for their generation. (e.g. The Desolate Era or Long Live Summons) Then there is OP like OPMan. He can do whatever he wants. I would say kind of like Arifureta. Or the Level 999 Villager. So What if it's an RPG World? Third Time in Another World. Then there is also "I am OP but I am cautious, or passive, so I won't abuse it and just mess around." This the The New Gate, or The Guild's Cheat Receptionist. Though the receptionist is a lot better than the gate. Evil God Average. Overlord. There is the other one where it's like, "I'm supposed to be OP and at times people treat me as it but for some reason, I still struggle so much in every fight." That is The Black Knight, or Isekai Maou to Shoukan Shoujo Dorei Majutsu.The Epic Tale of the Forsaken Hero. Last but not least, we can't forget about the OP ability but not really OP yet. Like Kuma Kuma Kuma Bear, she has an OP bear gear but she isn't OP, just her gear. But still, she isn't "strong" in the overall scheme of things yet. Legend is in here too. He's supposed to be super strong and OP but hasn't developed it out. A bunch of the Dungeon Master novels are kind of like this too. The DM has some cheat OP ability but they aren't OP yet or not crazily OP for the longest time till like near end of novel. P.S. We can't forget the OP MC that doesn't really do anything but builds a harem and have a ton of sex, aka plotless plots. Like the Zombie Master or any of those variations, I think there are two. Or The Story of Living as I Please After Being Reincarnated in Another World with a Ridiculously Cheat Power. Slave Harem in the Labyrinth. I like Kansutoppu! but I don't remember enough of it to place it.
the comments on Death March are hilarious too most of the time such as " Monsters with tentacles? Where my tentacle scene? Sand giant x squid >~< Dafaq"
Consolidated all that has been said so far, not checking any. added some links, if any of these are wrong reply and ill fix Hardcore Op-ness , To Love-Ru, boku no piko,The Last King ,the Forgotten Conqueror ,Long live summons, Martial god space, desolate era, Death March kara Hajimaru Isekai Kyusoukyoku (WN), Dragon Bloodline, Commushou no Ore ga, Koushou Skill ni Zenfurishite Tenseishita Kekka, LV999 Villager, Skill Taker’s World Domination ~ Building a Slave Harem from Scratch, Young God Divine Armaments, One-Punch Man, Yobidasa Reta Satsuriku-sha, Kumo Desu ga, Nani ka?, Magi’s Grandson,Sendai Yuusha wa Inkyou Shitai, She Professed Herself The Pupil Of The Wiseman, Spirit Migration, Tensei Shitara Slime Datta Ken, The New Gate, Tsuki ga Michibiku Isekai Douchuu, Hagure Yuusha no Kichiku Bigaku, What Came to Mind During My Third Time in Another World Was to for Now, Get Naked, against the gods, Arifureta, So What If It’s an RPG World?, The New Gate, The Guild's Cheat Receptionist, Evil God Average, Overlord, Isekai Maou to Shoukan Shoujo Dorei Majutsu, The Black Knight, The Epic Tale of the Forsaken Hero, Kuma Kuma Kuma Bear, Legend, Zombie Master, The Story of Living as I Please After Being Reincarnated in Another World with a Ridiculously Cheat Power, Slave Harem in the Labyrinth, Kansutoppu!, Elqueeness, Re:Monster, Konjiki no Wordmaster
literally any Wuxia, Xianxia or Xuanhuan falls under that category as they are all power creeps with mc being able to fight those at far higher levels however I personally recommend Renegade Immortal which is a pretty serious novel and my personal fav, A Will Eternal which is by the same author but more of a comedy with the occasional serious moment, Desolate Era which out of all the novels I've read has the largest scale of the universe as our entire observable universe is just a sealed part of the larger mortal realm which has 3000 major worlds each 10s of thousands if not even millions of times larger than earth which is only one of the 3 realms which is just a shattered chaos world which is then one of countless chaos worlds within a territory which is ruled by an everworld and then a cluster of terrirories is a realmverse and then a bunch of realmverses is a chaos verse and there are multiple chaos verses out there and in terms of time there are trillions of years within a chaos cycle and the mc lived for 6 million chaos cycles and he wasn't the oldest and beyond that some other recommendations are Library of Heavens Path basically mc is from earth but wakes up in some dudes body with a library in his head that lets him put any book he touches in it, King of Gods mc randomly gets the ability of an ancient god that created the world so he gets stronger and unlocks more of its power, Against the Gods which was my second and for a long time fav cultivation novel, Tales of Demons and Gods which was my first, while I personally didn't love it and dropped it at chapter 300 after taking a break and being too lazy to pick it back up and remember all the names a fan favorite is I Shall Seal the Heavens, tons of people recommend Reverend Insanity and its decent and if you want something on a smaller scale which is Wuxia then I recommend Legend of Ling Tian also sorry for the long comment but those are all things I recommend but TL;DR - Renegade Immortal personal fav and is more serious - A Will Eternal same author but is more light hearted and comedy - I Shall Seal the Heavens same author again and is really popular - Desolate Era largest scale universe I've read - Library of Heavens Path weird library system thing - Against the Gods my second xuanhuan novel and what got me into the genre - Tales of Demons and Gods my first xuanhuan - Reverend Insanity not my cup of tea but people are obsessed with it so I am recommending it for them - Legend of Ling Tian much smaller scale than the rest but still a good read and yes every single MC is insanely op and has all the plot armor so while they all struggle they are all able to fight those at far higher levels and I just now remembered another recommendation is Emperors Domination cause that mc is literally the definition of OP cause as a mortal he is able to defeat cultivators like 5 realms above him and once he starts cultivating he becomes strong enough to fight like the 10th out of 13 realms and eventually once he goes to the next world and needs to create a stronger cultivation system he doesn't even bother fighting that world for heavens will since he finds it beneath him