One common theme is friendship. The OC Angel might form a strong bond with the MLP characters, just like how friendship is emphasized in the show. For example, they could go on adventures together and learn important lessons about loyalty and kindness.
Another theme is self - discovery. The OC usually has some sort of journey where they learn more about themselves, and Luna also often discovers new aspects of her personality or her role in Equestria through her interactions with the OC. For example, in some fanfictions, Luna realizes she can be more approachable through her time with the OC.
The theme of acceptance is also prevalent. The OC may be different from the regular ponies in Equestria, and the members of the harem have to accept the OC for who they are. Also, adventure is a common theme. The OC and the harem often go on adventures together, which helps to strengthen their relationships and also provides exciting plot points. They might be exploring new parts of Equestria or facing off against villains together.
DeviantArt also has a section for fanfiction. Some artists write stories related to their MLP art, and there could be some with OC Angels in them. You might need to do a bit of digging through the MLP category to find the good ones.
One common theme is friendship. Just like in the main MLP series, the relationship between Rainbow Dash and the OC often revolves around learning about each other, being there for one another, and growing through their friendship. For example, in many stories, the OC might be new to Ponyville and Rainbow Dash helps them fit in.
Aria Blaze was going about her day in Canterlot when she saw a male oc passing by. His appearance was striking. Aria Blaze couldn't help but be drawn to him. She followed him at a distance for a while, until he noticed her. He was a bit shy at first, but Aria Blaze's friendly nature put him at ease. They started to exchange stories about their lives. The male oc told Aria Blaze about his travels, and she told him about her experiences in the music industry. Over time, their friendship grew deeper, and they became an important part of each other's lives.
There's also 'A Journey with Luna and the OC'. In this fanfiction, the OC is a magic user who is called upon by Luna to help deal with a dark magic threat. It has some really interesting character interactions between Luna and the OC. The story is full of adventure and mystery as they explore the origins of the dark magic.
One popular MLP OC x Harem fanfiction is 'A New Adventure in Equestria'. It features an interesting original character who finds himself in a harem - like situation with some of the well - known ponies. The story has great character development and engaging plotlines.
Once upon a time in Equestria, there was a beloved MLP OC named Starlight Glimmer. She was known for her kind heart and beautiful mane. But one fateful day, she passed away. Her friends gathered for the funeral. Pinkie Pie tried to keep the mood light with some songs, but everypony was still deeply sad. Twilight Sparkle gave a touching speech about how Starlight had changed their lives. The whole place was filled with sorrow as they said their final goodbyes to Starlight Glimmer.
Maybe the OC is an artist who can see the true forms of the artworks that Dark uses to transform. She becomes involved in his missions when she accidentally stumbles upon one of his hideouts. The OC could be initially distrustful of Dark but slowly starts to understand his motives as they work together to protect the artworks from the bad guys in the D.N.Angel universe.