One possible theme could be redemption. Maybe the OC, despite being considered 'ugly' in some way, helps Tom Riddle see a different path than the dark one he ultimately takes in the original story.
Another theme could be acceptance. The story might explore how Tom Riddle, who is often depicted as vain and obsessed with power and appearance, comes to accept and even be attracted to the OC who is not conventionally beautiful. It could be about looking beyond the surface. Also, there could be a theme of transformation, where the relationship between them causes changes in both characters, for better or for worse.
There could be a theme of self - discovery. The 'ugly oc' might be on a journey of self - discovery, and Tom Riddle becomes part of that journey. Maybe through their interactions, the OC discovers new strengths or aspects of their personality, and Tom Riddle also discovers something about himself that he hadn't before. This could all be set against the backdrop of the complex and often dark world of Harry Potter.
Another theme is the exploration of Tom Riddle's character. The OC serves as a tool to dig deeper into what makes Tom the way he is. Through his possessive actions towards the OC, we can see his insecurities, his thirst for power, and his disregard for others' feelings. Writers often use this as a way to expand on the already complex character of Tom Riddle in the context of fanfiction.
Redemption is a common theme. Since Tom Riddle later becomes Voldemort, many fanfictions with an oc adopting him focus on whether he can be redeemed through the love and care of the oc. Another theme is the power of family. The oc often tries to create a family - like environment for Tom, which is something he lacks in the original story.
One common theme is the struggle between good and evil. Since Tom Riddle Jr. is on his path to becoming Voldemort, the OC often has to decide whether to support his darkening ways or try to turn him towards the light. Another theme is the exploration of Tom's past. The OC may dig into his family history or his time at Hogwarts before he fully embraced the dark arts.
Love in the face of darkness is a big theme. Despite Tom Riddle's evil tendencies, the oc wife falls in love with him, and the story explores how that love can survive. Also, secrecy is often a theme as Tom Riddle has many secrets that the oc wife slowly discovers throughout the story.
Well, it can be a bit difficult to recommend directly as the 'ugly oc' concept is quite subjective. However, you could join some Harry Potter fanfiction groups on social media platforms. People there often share and discuss different fanfictions, and you might be able to find the 'tom riddle x ugly oc' ones among them.
One common theme is the exploration of power. Since Tom Riddle is power - hungry, the male oc often gets involved in his pursuit of power in different ways, either as an ally, a rival, or someone trying to change him.
One popular theme is the struggle between good and evil within the relationship. Since Tom Riddle is on the path to becoming Voldemort, the OC might try to bring out the good in him. Another theme could be secrecy. Given Tom's nature, the relationship might be hidden from others in Hogwarts. And a third theme could be the power dynamics, as Tom is a powerful wizard and the OC's own power or lack thereof can create an interesting dynamic in the romance.
Love and redemption can also be a theme. Sometimes the OC's love for someone or something makes them try to redeem Tom Riddle from his dark path. This could be a romantic love for Riddle himself, or a love for the wizarding world that makes them want to change him. It adds a layer of complexity to the relationship between the OC and Riddle.
One great 'Tom Riddle and OC Fanfiction' is 'A Darker Shade of Magic'. In this story, the OC is a powerful witch who enters Hogwarts during Tom's time. She challenges his ideas and there are a lot of intense magical duels between them. It's really exciting to see how their relationship develops from mutual suspicion to a strange kind of understanding.
The theme of mystery is quite common in these fanfics. Given Tom Riddle's mysterious nature, the oc often gets drawn into solving the mysteries around him. It could be about his past, his plans, or his real intentions. Additionally, the theme of friendship is also present in some 'tom riddle and oc fanfic'. The oc might try to befriend Tom Riddle, thinking they can change him through friendship, which leads to a series of events that show the complexity of such a relationship.