First off, stop trying to make them 'fit' perfectly. Canon purists will hate any OC anyway. The trick is making them feel like they could have been there all along, hidden in the margins. I sketch out a family tree or a social circle that lightly brushes against a main character—a childhood friend who moved away, a minor official in the capital, a merchant who supplied the heroes' gear. Their problems should be shaped by the world's rules, not the plot. In 'Naruto', a medic-nin OC isn't about becoming Hokage; it's about the ethics of chakra healing, the politics of the hospital, the burnout. Give them a skill set the mains lack, so their inclusion feels necessary, not intrusive.
Honestly, the most believable OCs are often the most mundane. Not every new person has to be a secret prince or a prodigy. A clerk in the Soul Society bureaucracy, a droid mechanic on a backwater planet in 'Star Wars', a minor noble's third son in a historical drama—their lower stakes make the world feel vast. Their compelling nature comes from how they react when the big canon events finally crash into their small lives. I once wrote an OC who was just a baker in the village below Hogwarts; the Battle of Hogwarts wasn't about fighting, but about sheltering refugees and keeping everyone fed, which felt surprisingly raw.