Apologies for 'bumping' an oldish thread, I am unsure of whether it has been answered in another thread somewhere else.
I can now confirm, I have had actual acknowledgement from O2 that their customers do take priority over Giffgaff.
Apparently, 100 users on a cell network (for instance) and both a O2 and Giffgaff customers are waiting in line, the O2 customer gains priority to the cell tower, the Giffgaff customer gets the *beep beep beep* and has to retry. On the second attempt, that Giffgaff customer then has the same priority as any O2 customer, meaning squeezing into the cell network should be easier, unless it's jam packed with O2 customers already.
Apparently though, those areas where signal is strong and is surrounded by lots of cell towers (a lot of places for O2), this shouldn't be a issue. It does become an issue however if there are any areas that are built up/largely populated and there isn't enough cell towers to support both sets of customers.
Hope this clears up any misconceptions for those of you who were not already positive.