Maybe an OC who is a long - lost relative of one of the main characters. This OC could come to the neighborhood with a secret that shakes up the relationships among the characters, like a hidden inheritance or a family feud from the past. For example, if they are related to Cesar, it could create tension between him and his gang - affiliated family as the OC has different values.
An OC could be a foreign exchange student who moves into the block. They don't understand the local culture and the complex relationships at first. But as they interact with the main characters, they start to learn and also introduce their own culture, which leads to some really interesting cultural exchanges and new friendships.
One idea could be an OC who is a new student at the school in 'On My Block'. They could have a unique talent like being an amazing street artist and use it to bring the community together for a local art project.