OC means Original Character. Basically like, your own characters, typically used in roleplays or stories, or just made because you feel like it.
In ES stream we post just about anything we want, doesn't have to be eeveelution related but a lot of it is, mainly just memes, artwork, lore about characters, or random pictures of Eevee we find.
You can also check the stream rules and scroll to the bottom there is a page with related sub-streams.