Dang, /O has been eluding me for months. Every now and then I try new things, but I really can not find the clue that will unlock it. I could unlock "Vault" and "Fell stroke", but knowing that there is a document AND a wav file behind /O... I guess I have to keep trying to unlock this last bit!
EDIT: Ok, just found /L and with this I got all 3 documents: "Fell Stroke", "Vault" and "Deep", plus "Item O". These are great! Nevertheless, password for /O I am pretty sure that it did not work first time I used it some months ago but now it did. Well happy to finally have all 3 secrets"