The only year I've even tried acoustic during the festival (never normally up that way) I had the paper wristband from Monday's bar crawl and got in okay. Have definitely heard it's hit or miss from Wednesday onwards though.
I've found if you're Oxfam, it's sometimes better to tuck your ID away and just let them guess at where you work. Stories floating around last year that some bars were letting EPO bands in but if they knew you were Oxfam they'd knock you back. Not 100% on why they'd do that, but that was the chat.
Second what most folks have said though, once the festival starts they're mostly good for slightly cheaper pints and a chill. Don't usually spend a whole lot of time in them