Fanfiction.net is also a good place. There are many writers who post their Sebastian Michaelis and OC fanfictions there. However, the site's search functionality might not be as advanced as AO3's, so you may need to dig a bit deeper. You can look through different categories related to the Kuroshitsuji fandom to find what you're looking for.
You can start by looking on Archive of Our Own (AO3). It has a large collection of fanfictions, and you can easily search for Sebastian Michaelis and OC fanfictions using the search bar. Just type in the relevant tags like 'Sebastian Michaelis' and 'OC' and you'll get a list of fics.
Some Tumblr blogs are dedicated to sharing and recommending fanfictions. You can search for tags like '#Sebastian Michaelis OC fanfiction' on Tumblr. These blogs often have reviews or summaries of the fics, which can help you decide whether a particular fanfiction is worth reading or not.
Chapter 461, "Different Answers", is the clearest early answer because it turns Damien's situation into a concrete plot event. The setup is The scent of roasted meat still lingered in the air long after the..., but the pressure keeps building through The clinking of silverware faded, leaving only the low murmur of.... Once the chapter lands on And you'll have both, as long as I draw breath., the chapter closes with Damien said nothing.
Chapter 367, "This Is What He's Made For", is the clearest early answer because it turns Damon's situation into a concrete plot event. It opens with The spiral gate to the Maze of Wills had opened like the mouth of... and closes on Three of the four circles flickered yellow.. The key payoff is that the chapter closes with Leana nodded. "This is what he's made for., which gives This Is What He's Made For a stronger narrative position and pushes the power growth onward.
Chapter 357, "A Second Demon", is the clearest early answer because it turns Damien's situation into a concrete plot event. Early on, [Kill Count: 2,027 / 5,000] [Combined Summon Kills: 1,154] Bang!. By the end, – Only targets deemed hostile will be burned and consumed.. Inside the broader family politics storyline, the chapter closes with Then it moved., which gives the main cast a stronger narrative position and pushes the power growth onward.
Chapter 355, "A New Participant", is the clearest early answer because it turns Arielle's situation into a concrete plot event. Early on, [Kill Count: 1,759 / 5,000] [Combined Summon Kills: 622] The ground.... By the end, I'll just keep killing till something pops up." ~~~~~ Arielle moved.... Inside the broader technology upgrade storyline, the chapter closes with A new participant had arrived., which gives technology upgrade a stronger narrative position and pushes the power growth onward.
Chapter 419, "I Didn't Lose", is the clearest early answer because it turns Lyone's situation into a concrete plot event. The setup is The faintest scrape of hinges reached Damien's ears., but the pressure keeps building through Through the slit of the half-open door, he caught the glimpse of.... Once the chapter lands on Lyone's chest heaved, his training evident in every strike., the chapter closes with "Hehe...
Chapter 415, "An Inside Attack", is the clearest early answer because the buildup finally cashes out in a real clash. A useful checkpoint is the way the scene starts with Apnoch shook his head, a wry smile tugging at his lips. and follows through with It was inside of Delwig, the impregnable city..
Chapter 300, "A Battle In The Testing Phase", is the clearest early answer because the buildup finally cashes out in a real clash. A useful checkpoint is the way the scene starts with All Year Three students walked toward the speed platform—a straight... and follows through with I hope they don't provoke me though." Damon murmured..
Chapter 385, "Tell Me Where", is the clearest early answer because it turns Razel's situation into a concrete plot event. Early on, The moon was high, almost reaching it's peak by the time Razel Acheon.... By the end, Draped in deep charcoal with silver trim.. Inside the broader family politics storyline, the chapter closes with "Tell me where. Now., which gives the central conflict a stronger narrative position and pushes the power growth onward.
Chapter 82, "Whether I Can Do It or Not Is My Business", is the clearest early answer because it turns Rogel Camp's situation into a concrete plot event. The chapter turns that into something concrete by moving from Leader, the number of people has been counted. to I won't say much more." Xia Hong did not say much, and after two....
Chapter 64, "Letting These Two Kids Off Easy", is the clearest early answer because it turns Solidified Fire Oil's situation into a concrete plot event. The chapter turns that into something concrete by moving from Hehehehehe....." The Wood Puppet Monster peered into the ravine,... to The Wood Puppet Monster was talking about the Solidified Fire Oil!.