Well, the 'teacher - student' trope can be seen in some Momonga x OC fanfictions. Momonga imparts his knowledge of the Overlord world and magic to the OC. The 'opposites attract' trope is also there. For example, if Momonga is all about order and the OC is more of a free spirit. And don't forget the 'betrayal' trope. Maybe the OC is forced to betray Momonga due to some external influence, but then they try to make amends later. These tropes can add depth and interest to the fanfiction.
One popular trope is the 'fish - out - of - water' scenario where the OC is from a different world or culture and has to adapt to the Overlord universe, with Albedo being a key figure in that adaptation. Another is the 'forbidden love' trope, considering Albedo's loyalty to Ainz. It creates an interesting dynamic as the OC tries to win Albedo's heart while she is devoted to another.
To write an engaging Momonga x OC Overlord fanfiction, first, understand the characters well. Momonga is complex, so study his traits from the original work. For the OC, create a detailed backstory that can interact meaningfully with Momonga's world. For example, if the OC is a mage, think about how their magic system will fit into the Overlord universe. Also, focus on the relationship between Momonga and the OC. It could start as a wary alliance and develop into something deeper. Use vivid descriptions to bring the story to life, like describing the dark and mysterious settings of the Overlord world.
One popular trope is the 'Powerful OC' trope. In many Goku x OC fanfictions, the OC is often created with unique powers that either complement or rival Goku's. Another is the 'Training Partner' trope, where the OC becomes Goku's training partner and through their training sessions, a relationship develops. And the 'Saved by Goku' trope is also common, where the OC is in some sort of danger and Goku comes to the rescue, which then leads to their relationship evolving.
One popular trope is the 'stranger in a strange land' where the OC is from another world or time and Link has to help them adjust. Another is the 'love at first sight' trope, which is quite common in many fanfictions. And then there's the 'OC with a special power' trope, where the OC has some unique ability that either helps or hinders their relationship with Link.
A common trope is the 'fish - out - of - water' situation. The OC, being from a normal world, is completely out of their element when dealing with Slenderman and his spooky realm. Another popular trope is the 'damsel in distress' or 'heroic OC' dichotomy. Sometimes the OC is portrayed as a helpless victim that Slenderman preys upon, while other times they are the brave hero who tries to fight back or outwit him.
One idea could be that the OC is a new player in the game who gets accidentally transported to the world where Momonga is. Momonga, being curious about this strange new entity, decides to keep the OC close. They then embark on adventures together, with the OC showing Momonga different perspectives from a normal player's view.
The 'mystery' trope is also prevalent. The xenomorph's origin or purpose in the fanfiction might be a mystery that the OC is trying to solve. It could be linked to some ancient alien prophecy or a scientific experiment gone wrong. And then there's the 'transformation' trope. The OC may start to take on some characteristics of the xenomorph, either physically or mentally, as a result of their interaction.
Well, a common trope could be the 'mentor - mentee' relationship in oc x Police Academy fanfiction. The OC might be a young and inexperienced recruit who gets paired with an experienced officer from the Police Academy as a mentor. This allows for a lot of character growth and development. There's also the 'found family' trope, where the OC forms deep bonds with the other cadets in the academy, like they become a family away from home.
One popular trope is the 'fish - out - of - water' story where the oc is new to the Avengers' world and has to adapt. Another is the 'romance' trope, like the oc falling in love with one of the Avengers. The 'mentor - mentee' relationship is also common, where an Avenger takes the oc under their wing.
One popular trope is the 'enemies to lovers' trope. The OC and Rayla could start off on the wrong foot, perhaps having different goals or misunderstandings. But as they are forced to work together, they begin to see the good in each other. Another is the 'fish out of water' trope, where the OC comes from a different background than Rayla, and has to adapt to her world, creating humorous and heartwarming situations along the way.