I take it you want to standby for an earlier flight as is the system when flying in the USA. Remember that this works different in Europe. Most European airlines will abide strictly to the fare rules in terms of changes. There are exceptions but very few.

In terms of OK flexibility I don't know as I have never tried it with them. Your best bet is to go to the airport and ask. I doubt that you will have any luck though, nor would you with many other European airlines...