You can start by looking on Archive of Our Own (AO3). It has a large collection of fanfictions, and there are likely many Cersei Lannister OC ones there. Just use the search function and type in relevant keywords.
There's also 'Cersei and the Unknown Heir'. In this fanfiction, the OC is a supposed heir to a powerful house and how Cersei deals with this threat and possible alliance is the focus. It's full of political intrigue which is typical of Cersei's storylines.
Power is a common theme. Cersei is all about power, and when an OC is introduced, the power dynamic becomes a central focus. Whether the OC is trying to gain power through Cersei or is a threat to her power, it drives the story. Another theme is loyalty. Cersei values loyalty, but also has a hard time trusting. So, the OC's loyalty or lack thereof can create a lot of tension. Love can also be a theme, though it might be a twisted form of love, given Cersei's nature.
In Cersei Lannister x OC fanfiction, the character dynamics can be very interesting. Cersei is a complex character with her own motives and desires. When paired with an Original Character (OC), it allows for new storylines. For example, the OC could be someone who challenges Cersei's power-hungry nature or perhaps becomes an unexpected ally. Their interactions might lead to power struggles, emotional turmoil, or even a strange sort of friendship. The key is how the author develops their relationship in the context of the 'Game of Thrones' universe.
To develop the male OC, start with his goals. Maybe he wants to gain power and sees Cersei as a means to that end. His appearance can also play a role. If he has a striking look, it might attract Cersei's attention. His relationships with other characters are crucial too. If he has a close friend or mentor, they could influence his actions towards Cersei. For instance, if his mentor warns him about Cersei's dangerous nature, he has to decide whether to listen or pursue his own desires.
One interesting storyline could be the male OC being a powerful knight who is initially sent to spy on Cersei but ends up falling in love with her. He has to navigate the dangerous politics of King's Landing while keeping his feelings a secret. Another could be that the male OC is a foreign noble who comes to Westeros and forms an unexpected alliance with Cersei. They face many challenges together, like dealing with the other noble houses' plots against them.
You should start by researching Cersei's character traits thoroughly. When introducing the OC, make sure they have a believable reason to interact with Cersei. Regarding the lemon aspect, it's important to write it in a tasteful way. You could write it as a passionate moment that reveals new sides of both characters. Also, consider the setting and how it can add to the mood.
One plot idea could be an OC who is a healer and tries to cure Cersei of a mysterious illness that no one else can figure out. This could lead to a complex relationship between them as Cersei is not used to depending on others.
To develop their relationship realistically, start with their initial meeting. If the male OC is from a different background, there should be some level of distrust or misunderstanding at first. For example, if he's from a poorer region and she's a Lannister, she may look down on him. As they interact more, small acts of kindness or shared interests can start to break down the barriers. For instance, if they both have a love for a certain type of horse, it can be a starting point. Their relationship should also be affected by the political climate around them. If Cersei's power is threatened, she may be more hesitant to show affection for the male OC for fear of looking weak.
Another place could be FanFiction.net. There are many talented writers who post their works there. However, the quality can vary. So, you might need to look through a few pages of search results to find really good Oc and Cersei Lannister fanfictions. It's also a good idea to check the reviews and ratings if available.