A common theme is redemption. Bucky's past as the Winter Soldier is often explored, and the OC plays a role in his journey towards redemption. For example, in many stories, the OC helps him face his past actions and find a way to move forward.
There's also 'Bucky and the Unlikely Companion'. This fanfic shows how an OC, who is a healer, helps Bucky deal with his past traumas. It's a very heartwarming story with some great character development for both the OC and Bucky.
The Winter Soldier is none other than Bucky Barnes. His transformation into this role was a result of manipulation and experimentation. Despite his past actions, he later strives for redemption and works to atone for his deeds.