As a kid, his parents never noticed. At the manor, and as a pureblood, you were supposed to keep things neat, organized and clean. Little Draco never had a book standing in the wrong spot at his shelf, never really was dirty and managed to keep his teeth very clean.
So, at the manor, no one noticed the counting of random things in day to day life, or the “bad numbers, good numbers.” And because no one asked, Draco didn’t tell and assumed everyone was like him.
At Hogwarts, the fellow Slytherin always thought it was because he was so very anxious all the time, probably because of the pressure his parents put on him. They assumed it was probably ‘just the way the Malfoys rolled,’ and no one questioned his behaviour in any way at all.
This all happened, while Draco started to hate the parts of him that were caused by the OCD. Other Slytherins sometimes preferred neat and clean, but they always called Draco obsessed. And also, over his puberty, Draco started to develop intrusive thoughts, dozing off more often, this causing him to snap easier and get actual panic attacks when he suddenly realized he might have left the door open, even though he knew he closed it.
And because no one said anything about it, and no one seemed to care, Draco carried on doing what he did, sometimes managing to distract himself by counting some more.
So it was only when Harry and Draco got together, that Draco actually was talking about it. Harry asked why Draco didn’t want four chairs around the table, even though it was too small for five, and Draco just stuttered and flustered and finally screamed ‘just because!’
And when Harry spent the day with Draco, he acknowledged the fact that Draco seemed to brush his teeth, magically or by hand, almost eight times a day.
Harry had picked up the signs pretty easily, taking him to a Healer (‘no, no, POTTER, for Merlin’s sake, I promise, there’s nothing wrong with me at all!’). And when Harry and Draco talked about it, the Healer too, said it was pretty clear Draco seemed to have OCD.
This meant, that after almost twenty-five years of not being able to talk about it, Draco could talk about his intrusive thoughts, finally realizing it wasn’t him who wanted to do those things. And that since so long, he had an understanding partner who gave him the space he needed, who was kind and loving about it, although not too focused on it.
And Draco really was one for labels, such as pureblood, or gay, or since a few weeks then, OCD. And it felt so good to know he wasn’t weird, gross or overreacting, and he couldn’t do anything about it, and that all those things were fine.