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 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 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.
A popular one is 'Harley Quinn and the Mysterious Female OC'. This fanfiction creates a mysterious aura around the female oc. As the story progresses, we see how she and Harley Quinn form an unlikely bond. The descriptions in the story are vivid, making it a great read for fans of the pairing.
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 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.
Give the OC a unique backstory. For example, if they grew up in the slums of Gotham but had a love for art, it could influence their actions and interactions with Harley. Maybe they use their art skills to create elaborate disguises or distractions during their escapades.
Power dynamics is another theme. Given Harley Quinn's complex relationship with power, especially in relation to the Joker, the male oc can bring a new element to this. Maybe he has his own power struggle within the DC underworld, and his relationship with Harley Quinn either complicates or simplifies that struggle. It could also involve themes of loyalty and betrayal, as in the world of DC, alliances can be fickle.