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.
It depends on their popularity and the creativity of fans. If people are really into their characters and potential relationship, fanfiction could definitely exist.
Since it's a fanfiction, it probably takes the well - known world of Harry Potter and adds its own twist. The 'power of oa' is likely the central theme. It could be that the fanfic author has created a new power or concept related to 'oa' and is exploring how it interacts with the existing magic, characters, and settings of the Harry Potter series. For example, it might change the way Hogwarts operates or how the characters interact with each other. It could also introduce new conflicts or alliances based on this new power.
In the drama " Ah Mai Enlists the Army," Ah Mai ended up with Shang Yizhi. Although the official supporting role in the original novel was Xu Xiu 'er, in the drama version, the male and female protagonists were changed to Shang Yizhi and Ah Mai. Shang Yizhi became Ah Mai's right-hand man and loyal lover. The two of them experienced countless hardships together and finally got together. Therefore, Ah Mai joined the army and ended up with Shang Yizhi.