Obviously Brit is a big fan of The Cranberries, as she used their song in the movie "Sound of my voice", and this show is called OA. In the song "Promises" by The Cranberries, there is a very significant repetition of the sounds OA, OA OA, lasting for 30 seconds or so, towards the end of the song. It really gets stuck in your head, and the way Brit's character was explaining to her bf what OA means, she started repeating it just like Dolores does when she sings, but said, it kinda losses its essence when she starts saying it. Im sure that, as Brit was listening to the song, the way Dolores sings OA, probably awoken some kind of a deep symbolism, or archetypal image in her mind, that she wanted that to be the essence of the hurt and physically damaged character, Prairie-meaning in her mind and soul, she was pure, and that sound somehow expressed that about her, hence the new name OA.