Dipper from Gravity Falls would also work well. Dipper's intelligence and his thirst for knowledge could be useful in an Ok Ko crossover. He could help Ok Ko figure out any strange phenomena that occur during their adventure. And together, they could face the mysteries and dangers of a combined Ok Ko - Gravity Falls world.
Definitely Steven from Steven Universe. His kind and curious nature would contrast well with Ok Ko's more action - oriented personality. Plus, their different powers would make for an interesting dynamic. They could learn a lot from each other, like Steven could teach Ok Ko about gem culture and Ok Ko could show Steven some new karate moves.
Finn from Adventure Time would be a great addition. Finn is a brave hero, and his friendship with Jake could add an extra layer to the story. Finn and Ok Ko could have friendly competitions in terms of fighting skills and bravery. They could also bond over protecting their respective worlds from various threats.
Definitely Enid from 'OK KO let's be heroes'. She has her own unique fighting style and a cool personality. In a crossover, she could team up with characters like Dipper from 'Gravity Falls'. Dipper's knowledge of the supernatural and Enid's combat skills would make for an interesting duo.
Enid is also a favorite. She's strong and has a cool personality, so in fanfiction she might be involved in some exciting battles or have her own sub - plot about her past or her relationships.
A crossover with Adventure Time would also be cool. Ok Ko could end up in the Land of Ooo. He might team up with Finn and Jake to battle some sort of evil force that threatens both the plaza and Ooo. They could exchange their different fighting styles, like Ok Ko's formal karate training compared to Finn's more improvised sword - fighting.
I've heard of a fanfiction titled 'Crossover Clash: Ok Ko vs Nexus'. In this story, there's a sort of conflict that arises between the two universes. It's full of action and adventure as the characters from both sides have to figure out how to deal with each other. It also has some really interesting character development moments for both Ok Ko's and Nexus' characters.
Rad would also be a common main character. He's KO's friend, and in the fanfiction, he might be involved in some sort of lemon - inspired adventure with KO. Maybe they go on a quest to find the rarest lemon in the world or something.
Naruto himself is often a very popular character in these fanfictions. His unique abilities and his growth as a character make him a great addition to any crossover story. He can bring his ninja skills and his positive attitude to the world of Ok Ko.
Maybe a corrupted gem that has the ability to control technology. It sees the world of OK KO as a perfect place to spread its corruption. It takes overobox robots and uses them to attack both the heroes of OK KO and Steven. This gem villain could be really powerful as it combines the strength of gems and the technology of theobox world.
In some fanfictions, OK KO could go on a journey to a different dimension. This would be full of strange creatures and new challenges. He'd have to use his wits and powers in unique ways to find his way back home while also helping the inhabitants of that dimension.
The mash - up of different settings. The Ok Ko world with its own set of rules and the Nexus world which is likely very different. When combined, it creates a really unique backdrop for stories.
Perhaps OK KO travels to Steven's universe and is amazed by the different types of gems and their abilities. He could learn from Steven about gem culture and in return, teach Steven some of his cool fighting moves. This could lead to them going on an adventure together to help a group of gems who are in trouble.