Agree on OSL. If it's for a private flight I'd never even consider taking an airport hotel (200 euro a night or so by default for the one closes to the terminal?). OSL doesn't have any benches to rest on at night. The only seating is found behind the security check. There is, as alternative solution, a very early airport express train arriving at 5-ish in the morning, departing 20 minutes earlier from OSL. You may consider a night in a hostel close to central station, which should set you back about 40 euro or so. Plus 25 for the train. Just ignore all the junkies, beggars and hookers you'll encounter on your way there.
For when it comes to sleeping in airports, I've done that more than I'd like to. Especially when I was a student I did traveling on extreme budget and then every free night was welcome.
The last encounter I had with sleeping at an airport was at FRA. LH couldn't make my connection and I got rebooked to the next morning. As LH didn\t have any hotel rooms available in any hotel nearby anymore, I just got an economy long haul blanket and pillow and a 'good luck' wish. I found a bench where I could lay flaat and where not too many people were walking by. It was downstairs from the check in area, almost at the end of the corridor. I managed to survivie and none of my stuff was stolen. The most annoying thing is the continuous messaging 'Achtung, Sicherheitshinweis'.
Another (at the time) LH joint was DUS. I caught a flight at 6 AM to OSL and wanted to sleep. There are quite a few benches near the (former) LH check in facilities. If these are taken, then just continue all the way down to area C via the shops and restaurants. There will probably be something empty at some point.
I also slept at DTM. I had a very early W6 flight to KBP at the time and I didn't feel like getting to a hotel or so. Just be on time at DTM as they'll close the airport for the night. They won't boot you when you're already inside. There are a few benches in the main terminal building. It was an ok night, also because I was the only one around, apart from a cleaner.
I had to sleep once at NRN, as I had a very early flight to Turin. NRN is literally packed with people who do a low budget experience. I managed to find a bench upstairs and make myself comfortable. NRN has been expanded, changed and refurbished a few times since, so I don't know how it's to sleep now. As FR is still the main airline, I guess there's still a huge overnight crowd at the airport, though.
One experience was at AMS, at three different occasions. The first occasion I was returning from VIE with SkyEurope. It was for work. The flight got delayed 12 hours as SkyEurope was grounded over not paying its bills. This made me arrive somewhat at 3 at night. As I had hand luggage only, I stayed airside and found a few very comfortable seats near Pier D. My boss had a good laugh when I arrived next day, being completely worn out. The other 2 occasions were caused by delay and by flying a charter flight with a crappy arrival time. In those cases I had to pick up luggage, which means that there's no way to stay airside. On land side there is a few comfortable benches between the shops, but those are most likely taken by the first sleepers. There is also a few shops and bars that close at night and offer some benches. For example the ice cream shop next to the Burger King. It's wood, but ok enough for a few hours. Nothing can be found on departure level, nothing can be found on even higher levels of the airport. You may actually be booted if you go there at night.
Another experience was at KTW at night. There are a lot of people trying to sleep here at night. Just don't look drunk, suspicious or like a tramp: you'll be booted instantly. It's hard to find a good place for sleeping here.
Even worse was my experience in the old KBP, before they had the new terminal. I had a very early departure, I arranged a taxi to the airport and I even had a hotel room. However, I had been warned that it might be a madhouse in the morning, so it was necessary to arrive a few ours early. This proved to be completely true. The place was packed like Times Square at New Year's eve. Unbelievable. And indeed, security took a while. There was absolutely no place to sleep for the remaining time before flight departure.
Furthermore, I had a nice overnight once ad MAD. I found myself at a bench near one of the exits on the lower level. No one bothered me, it was all in all ok.
And finally I had an overnight stay on STN. As goes for NRN, it's a FR place, which means lots of people on a total budget travel experience. Come early or there will be no more room for you. Stand up and your place is taken. Despite the rain, people were even trying to camp outside, because it was too packed inside. Never again for me!
And finally, I had almost one night to kill at DXB. The good thing is that somewhere in T1 there are some capsules for hire, which are small but comfortable and quite well isolated. The bad thing is that it's $20 per hour or so. I was there for 3 hours and had a nice nap. Alternatively you can also go to a hotel in T1 (and T3 I believe), although 200 euro per night is a bit much, especially when traveling on a budget. There are a lot of benches, especially when you get a bit further away from the food court and the tax free shops. It's not bad, but I can't imagine having a few hours sleep there (and still having all my possessions when I wake up. There's some weird folks out there).
Well, I hope the above is a bit useful for anyone.