Iron Man could also be a good pairing. There's a lot of history between Iron Man and Ultron. The OC could be caught in the middle of their feud. Iron Man might try to either reprogram the OC or use it to get to Ultron, creating a tense and exciting dynamic.
Another option could be Scarlet Witch. She has her own history with Ultron. The OC could interact with her in a way that either helps her overcome her past traumas related to Ultron or the OC could be influenced by her powers, leading to unexpected twists in the fanfiction story.
Vision could be a great character to pair with an Ultron Robot OC. Since Vision was created from Ultron's original plan, there could be a lot of interesting interactions. The OC could either befriend Vision or be in conflict with him, depending on the nature of the OC.
Perhaps the OC is a creation of a mad scientist who wants to use Ultron's technology to enhance their own creation. The OC then rebels against the scientist's control and teams up with Ultron for mutual benefit, leading to a series of adventures where they face various challenges together, such as fighting off military forces trying to destroy them.
Bellamy could be a great character to pair with an OC. He is a strong leader and has a complex personality. An OC could either be a friend who helps him make better decisions or a love interest who softens his tough exterior.
Farkas from the Companions would be an interesting character to pair with the Dragonborn OC. He's a strong warrior and has a simple - minded but kind nature. The OC could learn a lot from him about combat and the ways of the Companions. Also, Brynjolf from the Thieves Guild could be paired with the OC. If the OC has a bit of a roguish side, they could get into all sorts of mischief together in Riften and across Skyrim.
Thomas would be a great character to pair the OC with. He's the main protagonist, brave and determined. Their relationship could start as a friendship, with the OC helping Thomas understand the Maze better or vice versa. Then it could develop into something more romantic as they face the challenges together. Newt is also a good option. He's kind - hearted and a natural leader. The OC could provide him with new perspectives, and they could support each other in leading the Gladers.
Luffy from One Piece could also work well. Luffy's carefree and optimistic nature against Gajeel's more serious and gruff personality would create an interesting dynamic. Maybe they're on a quest to find a powerful treasure in a new world created by the crossover.
Superman could be paired with Obito in a crossover. Superman's pure - heartedness and incredible powers could influence Obito. They could work together to protect a city under threat. Obito might be in awe of Superman's abilities at first but then find ways to contribute with his ninja techniques.
In a crossover with the Fate series, Saber could be paired with Toguro. Saber's sense of honor and her noble fighting style would clash with Toguro's brute force and underhanded tactics. It would create a really interesting dynamic, whether they are on opposing sides or forced to work together for some greater purpose.
In a crossover with the Naruto universe, Sasuke could be a great character to pair with Illumi. Both are somewhat dark - natured and highly skilled. They could start off as enemies, with Illumi seeing Sasuke as a target or a rival in power, but then gradually develop an understanding or a strange alliance as they realize they have common goals in terms of seeking power and revenge in their own ways.
Another option is to pair the OC with Kotoko. They could be like sisters. The OC could be more level - headed than Kotoko and help her out in her relationship with Naoki. For example, the OC could give Kotoko advice on how to handle Naoki's moodiness, and in return, Kotoko could introduce the OC to all her friends at school.
A healer - type oc would be an interesting partner for Garmadon. This oc could be trying to heal Garmadon's physical and emotional wounds. Nya could also be a good pairing. She is intelligent and strong - willed. In a fanfiction, she could be the one who challenges Garmadon's ideas and helps him see things from a different perspective.