The Fem OC could be a reformed hero who is trying to stop Harley Quinn from her villainous ways. But as they interact more, they start to understand each other's perspectives and end up teaming up against a bigger threat in Gotham. This would involve a lot of character development for both of them as they learn from each other.
One great 'oc x Harley Quinn fanfiction' is 'Harley's New Ally'. In this story, the original character (oc) is a vigilante who crosses paths with Harley Quinn. They start off as enemies but gradually develop an understanding. The author does a great job of capturing Harley's complex personality and how the oc challenges her views. It has some really interesting action scenes and character interactions that make it a fun read.
A popular one is 'Harley Quinn and the Reader: A Gotham Adventure'. This fanfic is exciting as it takes the reader on a journey through Gotham's underbelly with Harley. The author's description of the city and the interaction between Harley and the fem reader makes it a must - read. There are some really intense action scenes and some heart - warming moments too.
A good 'oc x Harley Quinn fanfiction' often has unique plot twists. Maybe the oc has a secret that changes Harley's perception of them completely. Or perhaps Harley has a goal that the oc initially opposes but later comes to support. The story should also have good writing in terms of grammar and style. If the writing is sloppy, it can be really hard to get into the story. And the fanfiction should explore the themes that are relevant to Harley Quinn's character, like freedom, madness, and the search for identity, through the relationship with the oc.
Another theme is friendship. Despite their differences, Harley and the OC form a bond. It could start as a reluctant partnership but develop into a real friendship over time. Like in some fanfictions where they go through tough situations together and learn to trust each other.
One common theme is friendship. Often, the female oc starts as a stranger to Harley Quinn and they gradually become friends. They support each other through various challenges in the Gotham underworld.
One plot idea could be that the OC is a new villain in Gotham. Harley Quinn becomes yandere for the OC because of their unique style of chaos. She starts to stalk the OC, leaving little gifts that are both disturbing and endearing, like a dead bird with a love note.
For Harley, her yandere trait could develop as a result of her constantly being betrayed in her past relationships. When she meets the OC, she sees them as someone she can finally have complete control over, so she becomes overprotective and possessive.
Yes, there are likely some out there. Fanfiction platforms are filled with all kinds of creative crossovers. You can try searching on major fanfic sites like Archive of Our Own or FanFiction.net.