I wouldn't count on lounge access, as OK have made arrival access a published benefit of OK+ platinum. If you were OK+ gold, I could see them letting you in after telling them that you need to wait for someone else etc....With another card, I think your chances are quite low.

If you're arriving during business hours, get outside the terminal and go to the business centre behind the Courtyard hotel. On the first floor, there is a self-service restaurant (more like a canteen, actually) that's very popular with airport workers. I have never eaten there, just had drinks, but it's always packed and buzzing, and the beer is cold and cheap, so that should already be a good start. If that's not your thing, the ground floor boasts a very nice, very civilised Italian cafe-bar whose coffee is a million times better than what's available in the lounge.

If you're arriving in the evening, the above may be closed, but the airport is full of decent catering options with plenty of seating (both airside and landside).

Having said all that, if you are actually going to Prague, I would also be thinking about heading into town straight after arrival. It's not Tokyo- we're talking pretty short distances here. Outside of rush-hour, it's only 20 minutes in a (reasonably-priced) cab, or about 45 by bus+metro combination. Then you can wait for your mates in some trendy bar or at least a comfy hotel lobby...