I like 'A Different Kind of Magic'. In this fanfic, the male OC is a transfer student from a different wizarding school. Snape is assigned to be his mentor. The story is full of little moments where you can see the chemistry between them develop. It's not rushed, and the author does a great job of showing Snape's complex personality and how the OC manages to break through some of his tough exterior.
One great Severus Snape x Male OC fanfiction is 'The Unlikely Bond'. It explores the relationship between Snape and a new male student at Hogwarts. The story delves deep into how they initially clash due to Snape's gruff nature and the OC's headstrong attitude. But as the plot progresses, they discover common ground in their love for potions and a shared past of being outsiders. It's a really interesting read that develops their relationship in a very believable way.