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.
Fanfic often explores the idea of Bucky's metal arm sensitivity in relation to his overall state of mind. If he's in a stressed or emotional state, the arm might become more sensitive, almost like it's reacting to his internal turmoil. Writers may use this to show his vulnerability. For instance, when he's having flashbacks of his past traumas, the metal arm could tingle or ache, emphasizing the connection between his mental and physical state.
One great 'Bucky Barnes Metal Arm Fanfic' is 'The Arm and the Man' on Archive of Our Own. It delves deep into Bucky's relationship with his metal arm, how he copes with it both physically and mentally. Another is 'Metal and Memory' which shows his journey of accepting the arm as part of himself.