bruisepristineI flew over for the weekend last year but only went to the festival on the sunday (stayed in Dublin and did other things for the rest of the time I was there) as the line-up only really appealed to me on that day.
I've heard bad things about the campsite but I don't think it's really any worse than Reading/Leeds if you've ever been there. Obviously I only saw the arena and not the campsite, but the arena was great. Brilliant layout, easy to get around different stages (of which there are many, far more than most festivals), the stalls and food vans were diverse, the toilets were clean (yes, and this was on sunday morning :P).
The sound at the main stage was dodgy at times, and they had some problems with the big screens. Generally the sound was better the closer you were to the stage. It was really quite bad by the big tripod sound tower things (though bear in mind the weather was horrible, so wind probably didn't help the matter).
They do have a golden circle at the front (main stage), so if you want to be close, be prepared to queue! (I queued for about half an hour to get into it for Placebo, who were fourth from top, to give you an example). If you wanted to be right at the front for the headliner you'd probably need to go in a few hours before and just stay there. Although I'd have liked to see RHCP from the front I didn't want to watch Kaiser Chiefs and after they'd finished I just didn't attempt to get back into the front section (it's really not very big at all, Reading's golden circle last year was much bigger and also didn't require queuing).
Erm..not sure what else to say really. The line-up for Oxegen is usually fantastic but it's not necessarily going to compare, in terms of atmosphere, to other festivals. People usually have a lot to say against Oxegen, some of which is probably a bit unfair, but obviously I can't comment on the campsite. If riots and things are going to scare you I'd advise booking yourself into a hotel and getting a bus to and from the site each day (this is really easy and also quite cheap). It'll also mean not having to fly over with tent and other camping equipment.
Hope this helps. Feel free to ask questions if there's anything else you think I might know the answer to!