Well, without more context, it's hard to say exactly. 'oa' could potentially be an abbreviation for something unique within the fanfiction world, like a magical object, a spell, or an organization. Maybe it's something the author created specifically for their story.
As mentioned before, without clear definition of 'OA', it's difficult to say. But if it's Open Access, one main element is the fight against the traditional model of academic publishing where access was restricted. Another could be the role of technology in enabling open access platforms.