A possible plot for 'the flash barry x oc fanfiction' is that the OC is a meta - human with the power to create illusions. At first, Barry is suspicious of the OC, thinking they might be using their powers for evil. But as he gets to know the OC, he realizes they are just lost and scared. They start to work together, and the OC uses their illusions to help Barry in his battles against other villains. Along the way, a romantic relationship develops between them. Also, the OC could be hunted by a secret organization that wants to use their powers, and Barry has to protect them.
One idea could be that the OC has a unique power that complements Barry's speed. For example, the OC might have the ability to manipulate time on a small scale, and together they can prevent disasters that even Barry alone couldn't stop. Another plot could involve the OC being from a different dimension and Barry has to help the OC get back home while falling in love along the way. Or perhaps the OC is a scientist who discovers a new form of energy that can enhance Barry's speed even further, but there are some dangerous side - effects that they have to deal with.